I agreed with Merle but I understand Rick more
I think the reason Rick didn't kill the Gov outright is twofold:
1) When he realized the Governor had gotten there before him, he couldn't be sure if he was alone or if there were others hiding in the barn waiting to take him out.
2) He wanted to at least try to solve things peacefully.
plus he realized the governor has been following his every move, when he said they brought back quite a stash