They are serving the devil who has already lost! His loss is already baked into the cake. He has no chance of winning; he's just pounding the air and going through the motions. His whole existence is hopeless. He has nothing to hope for or look forward to, and the duped people who follow him aren't much better off.
Yes, he has already lost, but he still drags most souls to hell to lose with him.
But he can't even be happy about the number of souls he snatches from God. Each soul will add to his own eternal suffering.
His lot is still a miserable one.
Imagine you caught your wife "in flagrante" in the state of adultery. You've basically lost your life right there. No more happy family life, etc.
Now the devil does the only thing he can do -- act out on his envy, and try to ruin other souls.
Likewise, the spurned husband could get out a gun and kill the strange man and/or his wife. But would that REALLY make him happy? The man is going to be unhappy afterward either way. Having killed his adulterous wife won't make him feel any better, really. His life is just as ruined.
In fact, what he thinks will help him feel "a bit better" actually makes things worse. After all, a man will likely follow it up by shooting a 3rd person -- himself. Sometimes the children of his ruined marriage are murdered as well. But since murder is not approved by the State even if it's motivated by punishing adultery, the man faces lots of prison time (along with all the associated violence and guard-on-prisoner and prisoner-on-prisoner rape) if he doesn't commit ѕυιcιdє.