When it seems his past will destroy his future, a successful young man turns vigilante.

Hi!
I’m not a pro in logline so this is just my opinion.

Using the verb “seems” feels unclear and it feels like your logline is missing a real clear action. I don’t understand the reason why he turns vigilante. What is going to change for him by doing that?

I would say: When a successful young man realizes that his past will destroy his future, he turns vigilante and helps/ do something…. in order to ….

something like that.

Hope it helps 🙂