Gaster has multiple reasons for taking on his whole terrible secret project, and here’s one of them!
Gaster this is the exact opposite of
what Asgore would have wanted.
(Cool motive, still murder)
Gaster is very emotionally invested in Asgore (and Toriel as well, although
he currently thinks she’s dead) and completely unobjective when it comes to either of them. A bit similar to how the brothers just refuse to hear insults, haha.
Could this conversation have played a part in Gaster deciding not to choose Mercy? Hmm!