When masked men enter his home, and his wife is tragically killed in the crossfire, Jack knows his life will never be the same again. His undercover cop career over, he is given a new identity under witness protection and shipped to the middle of nowhere. Battling his guilt he decides to go against his new protection to drew out the killers and gain revenge.

Your logline has a lot of words for a simple concept.

"After his wife is murdered, a former undercover cop seeks vigilante justice against those responsible."

Good advice above additionally I would say better to isolate exactly which event it is that makes him want to take action. In this instance the interesting action would be revenge so the better to choose his wife being killed over his cover being blown.

How about:
After his wife is killed by a crazed masked man during a home invasion a guilt ridden under cover cop goes after the killer.

Hope this helps.

There are a couple things that can be easily done to improve this logline.

First, remove all the extraneous information not directly related to the throughline of the story. Phrases like Jack knows his life will never be the same? are unnecessary and just cloud the message you?re trying to get across ? what is this story about, and why should I want to read this script?

Second, unless your MC is a famous person, you don?t need to include his name, it just clutters the logline. Using some descriptive terms to describe the MC, such as a guilt-ridden undercover cop? will make your logline more efficient.

Here's my take, it needs some work, but hopefully it points you in the right direction:

"After his cover is blown and his wife tragically killed by vindictive enemy forces, a guilt-ridden detective comes out of witness protection in order to draw out those responsible for her death."