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 :-)

This second attempt is much better, now we have a better idea of hi his dark past

How about this as an example:
-----
When it seems his past childhood sexual abuse will destroy his future (as a [blank]), a successful (job title) turns vigilante (Against those who [blank])?
-----
Just curious, does he go after those who originally abused him, or does he go after those who he believes are hurting children in the present?

Thanks R. I'm actually a little torn between keeping it vague and getting the how and what down - on account of the subject being very dark.

What are people's reaction to

"When it seems his past childhood sexual abuse will destroy his future, a successful young man turns vigilante to exorcise his demons"