I would highly recommend reading The Order of Genocide, by Scott Straus, if you want to learn more about the topic. While its more sociologically focused than other books on the topic, and thus a little more dry, Straus went to great lengths to talk to both victims and perpetrators in order to figure out what happened. In doing so he manages to expose how normal, usually tolerant people can be pushed to do the unthinkable. The book is not only brilliantly researched and deeply interesting, but it also provides crucial, albeit terrifying, insight on how genocides like the one in Rwanda could happen almost anywhere given the right circumstances.
Now I'll start by saying that I'm neither an expert on global politics nor on the Rwandan genocide, but having had the opportunity to learn a little bit about both, I would love to try to change your view. Now before we jump in, I want to be clear in saying that I think you have the ethical high ground here, and that the genocide was allowed to happen represents a huge moral failing on the part of many Western nations who should have intervened. That having been said, this was a complex situation, and that makes assigning consequences after the fact a difficult process. To explain why, let me try to respond to what I see as the three main parts of your view.
Nations Pulling out of Peacekeeping Before the Genocide:
The fact that nations involved in the UNAMIR peacekeeping mission in Rwanda pulled out just before the genocide is horrific, especially since we know now that early intervention might have stopped some of the violence, but we have to remember the context in which this decision occurred. The peacekeeping force, which was made up mostly of Belgian soldiers, we undermanned, under-equipped, and set up with an overly limiting mandate from the start. They were so constrained that, even on the eve of the genocide and with the intent of Hutu extremists having become startlingly clear, they were scolded for requesting to raid known weapons caches, as this was seen as outside the legal bounds of their mission. As a result of these issues, when the genocide and death squads began targeting moderate Hutu politicians, the Belgian peacekeepers weren't sure if they were able to respond with deadly force, even after they had been in combat for over two hours. As a result, 10 Belgian commandos guarding the moderate prime minister voluntarily handed over their rifles to Hutu militia, and were murdered almost immediately afterwards. Seeing this level of dysfunction, some of the nations involved in UNAMIR reasonably began to question whether their efforts were worth the risk to their troops, and they may have begun to fear a repeat of the disastrous UN peacekeeping mission in Somalia, which had been doing nothing to stop the violence there. With all this in mind, pulling troops out may at the time have seemed to be the only reasonable decision, as the nations doing so may genuinely have thought a continued presence in Rwanda would put their troops at risk without generating any benefits for the Rwandan people. Again, with hindsight we know this was a terrible decision, but we have to look at this choice with only the information available at the time.
Adding to this, have to look at the geopolitical consequences to your proposal that we punish nations for pulling out of UNAMIR. While some form of punishment is certainly appropriate from a moral standpoint, it might actually make future peacekeeping and genocide prevention all the more difficult. If nations know that they may be punished for leaving a UN mission they see as unsuccessful, they'll likely be less inclined to participate in the first place, severely limiting the UN's power. Given how difficult it already is to gather an effective peacekeeping force, handing out punishments could inadvertently lead to a situation in which the UN would be left impotent to stop future genocides.
France's Support for the Hutu Government:
On this point I would say you have the strongest case, since France's decision to arm and train the Hutu government's troops was a pretty amoral one even before the genocide, but yet again we run into problems when it comes to enforcement. As bad as France's decision making was in this case, they couldn't have known with certainty that the weapons and training they provided would be used to carry out a genocide. Similarly, while the Habyarimana regime was morally repugnant, France was supporting it legitimate in order to reinforce its geopolitical position. If we were to punish France for these actions, it sets up a precedent that would drive many powerful nations into open revolt. No major power is going to agree to a framework for punishment that penalizes the sale of arms that later go on to be used in human rights abuses. Similarly, no major power is going to accept limitations on which governments they are or are not allowed to ally with, as this would be seen as an affront to their sovereignty. Although again I agree that punishing France is the morally right thing to do, the resulting backlash from many major global powers would be a huge blow to international diplomacy, which again would make future peacekeeping and genocide prevention all the more difficult.
The Rwandan Justice System Post-Genocide:
So for this section I'm going to be working in part from The Order of Genocide, by Scott Straus, which does a really great job explaining how the genocide unfolded. Essentially, what Straus discovered is that when the genocide started groups of extremist Hutu militia members spread across the country, conscripting civilian Hutus into killing mobs after disabling local law enforcement that might have stopped the violence. Some of these Hutu civilians went willingly, but many only did so under threat of physical harm or having their property destroyed. Other Hutu civilians only engaged in the killing because they were sheltering Tutsi friends or relatives in their own homes, and felt that refusing to take part would look suspicious, putting their lives and the lives of those they were protecting in jeopardy. As a result of this, a significant proportion of those who participated in the genocide did so under duress, and not because they believed in Hutu supremacy or hated Tutsis.
On the practical side, Rwanda simply couldn't handle pursuing full criminal cases against everyone involved in the killings. Post-genocide, about 1/5 of the entire population was estimated to have taken part on some level. From the start arresting all of these individuals would have been logistically unfeasible, and the sudden loss of manpower would have further destabilized Rwanda. Even when the new government only arrested the 100,000 people who most actively drove the genocide, this still led to severe prison overcrowding and inhumane conditions for prisoners, some of whom were likely innocent. Deepening this problem, Rwanda lost over half of its judges during the violence, and as aforementioned the extremist Hutu groups who initiated the genocide often destroyed local law enforcement and judicial institutions which were subsequently needed to bring them to justice. This lack of resources made the already daunting task of punishing every Rwandan involved in the genocide essentially impossible.
With all this in mind, while the reconciliation focused justice process following the genocide wasn't perfect, it was probably the best option left. This method gave the chance for survivors to receive a public apology and a chance to have what they suffered through recognized, which helped to deal with the collective trauma inflicted by the genocide. This also meant that those Hutus who were coerced into killing were given at least some form of a repercussion, but were also allowed a degree of leniency in acknowledgment of the fact that they likely would not have been violent without being pressured by outside forces. At the same time, this freed up the Rwandan justice system to pursue more serious criminal charges against those Hutus who either initiated the genocide, actively recruited people into the killing mobs, or showed a particularly high degree of cruelty and intentional violence during the genocide. Although this arrangement was imperfect, it helped to adequately punish those who bore the greatest responsibility for the genocide, while also allowing Rwanda to move forwards as a functional country.
Anyhow, I hope all this has helped to give you a new perspective on this issue. Let me know if you have any questions, as I would be more than happy to discuss the matter further!
EDIT: Typo