Was Foggerty working for Richie?

Yes, he was working for Richie, but kill who?

Seize the moment, 'cause tomorrow you might be dead.

a. There'd be no movie.

b. Richie's task was to atone... to "make things right." If Fogerty had killed Joey in Indiana, Richie wouldn't be able to offer up Joey to whoever the big guys were that were demanding restitution... or whatever.

c. If you mean, why didn't Joey kill Fogerty in Indiana, it's because he didn't want to tip his hand that he'd been lying all this time. Also, he didn't want to HAVE to go back to Philly, and maybe get killed there... or worse!

For dramatic purposes, Tom has 3 things to fix from his past:

1) He didn't kill Fogerty when he could've, so that mistake is now haunting him.

2) He thinks he "killed" Joey, but he hasn't. He still is violent inside.

3) He has built his new life on a lie. This has sown the seeds of growing problems. E.g. he has taught his son to be a pacifist, but that also makes him a target for bullies.

"Why not just kill him? from start?"

Because Richie wanted to see him first.