Such an idiot, an absolute failure of a yandere even.
Once you realize poison doesn't work, you don't exhaust yourself to keep trying poison. You wait for a chance to "accidentally" catch the wife in an explosion. Then you play up your "self loathing guilt" in order to win some pity points to worm your way into the man's heart.
We know that wouldn't work out like that, obviously, but that is absolutely the course of action a competent yandere would have shifted to.