Opinion: Giving villains a backstory on why they become monsters isn't excusing them for their actions. In the hands of a bad writer, it can come off as a poor justification for their actions. But, in the hands of a good writer, it's nuanced, giving the villains depth and showing us their motivations for why they act in a vile manner.
Explaining why the villains become the man/woman in the present doesn't mean they are forgiven/absolved for their heinous acts toward everyone in a town, village, city, planet, or otherworldly realm. You can understand why they are in that state while not defending them. Granted, even if there are some truths to what they are saying, you can understand where they are coming from, not defend them.
Here's an example: Donovan was born with superpowers; he was abused by his parents (without superpowers) and bullied by peers (without superpowers). Over the years, he grew angry and resentful of those without powers and took it upon himself to teach them a "lesson" like they were doing to him. From this day forth, Donovan decided to teach everyone without superpowers a lesson on Earth.
In all honesty, the type of villains that have these backstories, who're also not using their pasts as excuses and acknowledge them, and admit they'll lose or die, are the scariest ones.