If papa Pert had served beside people who weren't insufferable assholes or been sent to fight anything aside from basically the entirety of all Hrud that have ever existed throughout time he probably wouldn't have become a bitter bag of dicks. In Master of Prospero he's just as calculating and cynical as ever, but he's a good guy. He's evacuating a planet that's about to die and he knows he can't save everyone, so he uses the skill he's best at, logistics, to save as many people as possible well not getting emotional over the rest. Fighting the Hrud would have fucked up any Legion, because beating them is basically an exercise in futility. The Iron Warriors suffered more that most would have because Perturabo didn't know when to quit.
If he'd just been stuck with Magnus or fought anything other than time travelling aliens who cause reality to decay and are basically immune to most forms of weaponry he likely wouldn't have cracked. Hell he turned because of how brutally he put down Olympia. If it wasn't for the Hrud killing so many Iron Warriors Olympia wouldn't have lost so many of its sons and therefor wouldn't have rebelled.
Fun fact: Rogal Dorn is a Hrud.