A father seeks revenge on the internet troll who might have been responsible for his teenage son’s suicide.

Good re write, however it needs more structural work and trimming:

"...To exact deadly retaliation?" = kill, no need to beat around the bush with this it's better to be clear in what he wants to achieve. I also find that killing the troll sounds a bit harsh, perhaps give him a more nobel goal such as preventing further deaths.

"...finds himself?" = a passive protagonist, no need to describe the mid act 2 reversal just his grand plot objective.

Here is a refined version:
After his son commits suicide, ?a grieving father must pose as a teenage girl online in order to?catch the internet troll responsible for his son's depression and stop him from hurting other children.

Great feedback all. You guys are fantastic.

How's this as a revised logline:To exact deadly retaliation on the cyber bully responsible for his son's suicide, a father poses a girl on the internet. The father finds himself engaged in a deadly cat-and-mouse game, soon realizing that he was the bully's target from the get go.

>>>posses as an 16 year old girl on the internet to entice then trap the cyber bully

And then what? ?What does the father intend to do after he has entrapped him? ? The posing is a means to ?his objective goal, but it is not his objective goal. ?The objective goal is what he wants to do once he's entrapped the troll.

Not that the logline should give away how the story will end. ?It should not reveal whether he succeeds or changes his mind. ?But it should indicate what he wants to do -- his objective goal.