Nemesis

Chakotay's shuttle crash lands on a planet where a war is being waged between two species. He is drafted, brainwashed and trained to hate and kill the enemy.

The one species is contacted by Voyager, and helps find the missing first officer.

Tuvok goes with the aliens to rescue Chakotay, who sees him as an enemy alien
until Tuvok is able to convince him it is he.

At episode's end, Chakotay is confronted in sickbay by one of the aliens, who look kinda like Nausicans.

He is unable to relate to the alien and leaves sickbay.

He later tells Capatin Janeway that he wishes it were as easy to stop hating
as it was to start hating.

This episode was kinda preachy, but it made a good point:

Being brainwashed to hate is bad.

I used to not like this one, but, after seeing it a few times now, it is pretty good.

"I offer you a Vulcan prayer, Mr Suder. May your

death bring you the peace you never found in

life." - Tuvok.