I get what you're saying, but you say it like the timeframe doesn't exist, or that it's 'lacklustre' for a character (originally of good intent) to be worn down and converted to the bad side after a near-century of being soul-crushed in an unwinnable battle. I can see why someone would become dishevelled and eventually buy into the concept of 'peace' (despite it being provided via horrible means) when discussing someone who has endured nothing but loss and despair for practically 100 years of combat. If you can't, that's fine, so I'll just repeat my previous comment:
Remember, Ran's most commonly used words are "Peace" - while regarding us as the "Villains" for taking away said peace (however corrupt) from a child. Nobody is going to defend or justify his actions or motives, but they are easy to understand. Try to imagine fighting the Sin-Eater wars for decades on end, training the closest thing to a saviour time and time again and repeatedly watching them die, until finally, some semblance of peace is actually established (by a different means). I'd feel no pity if this was some random pleb who wanted a piece of that Eulmore peace-pie and threw themselves at it without thought, but we're talking about someone who (at one point) did fight for the right cause for practically a whole lifetime being withered down by it.^ Above mentality discredits the untold number of years/generations he fought against the Sin-Eaters, training those Minfilia's and seeing them cut down, time and time again. This wasn't some rando who saw Vauthry and immediately bent the knee - this is a chap who was likely Numero Uno in the Sin-Eater slaying department on the First who had nothing to show for almost a century of fighting but constant defeat (or as close to it as you can get without being wiped out). Then Emet comes along, instigates the birth of a human Sin Eater who can control those beasties. You'd be tired too after all that time and far more susceptible to the notion that maybe throwing your lot in with that obese scumbag would be preferable to fighting for the losing side which now also has people defecting/mind-controlled and bleeding out numbers without even fighting at all. If not for himself, then for the other Minfilia's.
But anyway.