The rate at which they perform self-actions can be surprisingly high, and can happen at the absolute worst of times. Yes, a Rapturous Flagellent and certain Crimson Curse states give a 'statistical' buff - but there is NOTHING good about a situation where characters can randomly decide to perform actions themselves. Really wish people would stop branding buff/self-turn states in a positive light.

It also makes him to have some bad 'behavior' sometimes like, attacking by himself, hitting someone, etc.

The only response to reaching 100 stress for this guy is becoming RAPTUROUS, which gives him crit and damage, and nerf dodge. He also cannot partner with additional Flagellants, whereas you can take 4xAbominations if you wish (who have their own hidden balancing via the anti-religion state - which may change in the Farm DLC). From my understanding he also cannot engage in any stress-relief other than Penance Hall (which makes total sense and isn't too detrimental in the grand scheme of things). You can essentially call it hidden balancing.

Darkest dungeon virtue meme