Here’s an interesting thing - Resurrection of the Daleks was intended originally for the end of Season 20 and didn’t feature a departure for Tegan. The script was re-written slighly for the following season to write out Kamelion’s scenes (having joined in The King’s Demons, he would have featured in the next story) as by Season 21 they realized that the robot didn’t work well from a technological basis. And the script was also re-written for Season 21 to write out Tegan. Thus, one could make the argument quite successfully that her departure scene is “tacked on”.

Technically it is tacked on - but it doesn’t feel that way, particularly if you are watching the story without the behind-the-scenes knowledge. When Tegan says she has stopped having fun because there’s been so much killing, it is not as though the audience shouldn’t believe her. Resurrection of the Daleks has the highest number of on-screen deaths for any Doctor Who story (and by that I am referring to a body-count - ie. living beings who are seen to die on the screen, rather than the reported deaths from planets, galaxies or continents that occurred in stories like Logopolis or The Parting of the Ways).  Tegan is the only member of the TARDIS trio who doesn’t advocate murdering someone in this story. Seeing the Doctor get pushed over the edge (by the Daleks plan to invade Gallifrey) resulting in his attempt to try to kill Davros in cold blood (something that she probably assumes he succeeded in doing since she wasn’t there to see the Doctor fail to do it and only sees him afterwards completely un-harmed) likely affected her as much as the horrible death of the man she called for help (who was minding his own business with a metal detector and didn’t even hear her, but still gets brutally shot in the back) - not something which is her fault but at the same time you could see why she would feel awful about it almost wishing that she hadn’t tried to escape after all.

In the end it is an effective, unique departure which works well, even if arguably it is one of the more depressing and bleak reasons a companion chooses to leave. But the Doctor says he must mend his ways as a result of Tegan’s departure and the next time we see him in a very bleak situation surrounded by people firing guns left right and centre (two stories later in The Caves of Androzani), he doesn’t join in at all - he rises above it and just has one thing on his mind, which is to save Peri. He essentially sacrifices his 5th incarnation (and for all he knew, his own life) to save someone who he didn’t know that well yet, but was in his care. In that sense, Tegan’s departure is not just an effective departure on its own, but it also sets things up well for the 5th Doctor’s own departure. Not bad for a script that originally wasn’t aiming for this at all when it was first written.