Re: You cant kill Units, How about retraining them?
Could you make it as a plain png file, without background? Similar to other buttons (could be found in GUI.rx after export from F11 menu).
Also it would be nice to have new thought cloud above retiring unit head. f.e. it could be bag on a stick, similar as it is on your button image.
Examples of thoughts are in Units.rx, from 3_6251.png to 3_6314.png
Preview (as gif):