Undertale Discussion

I like that idea a lot! However, there is a few tidbits that I would like to add in to this theory, if that is alright.

I too believe that Gaster was the Royal Scientist before Alphys, and he was the one that built the true lab. I also believe that he was the one that STARTED conducting all of these experiments in his almost crazed attempt to free the monsters from the Underground. I also believe he was the one that built the machine behind San's house, but that wasn't what led to his demise. It was determination.

Let's assume for a moment that Sans and Papyrus are somehow related to Gaster in one way or another, as it is popularly believed. We also should assume that Gaster was the one that build the Core, which is what provided power to the entire Underground, and, more importantly, his Lab. We should also assume that this trio of skeletons are old. Very old. When the first human fell to the Underground, Gaster was already far into his experiments. However, with the addition of Chara, Gaster found a new source. Determination. I would like to believe that, somehow or another, Gaster was able to take a portion of Chara's determination. This may reflect as to why Chara became so... murderous. Especially when before in the pictures in the game Chara was a part of the family with Asgore, Toriel, and Asirel. It could be written off as happy, if Chara's face weren't hidden in those pictures. However, there is actually no real proof for any of that. It's just an idea I built in my head.

But enough about Chara, for now. If we want to understand what happened to Gaster, at least based off of that theory, we'll have to look at a couple of characters that are much more recent. Undyne, for example. It is proven that she has her own determination when you kill her in either the genocide or the pacifist route. It's what causes her to cheat death, allowing her to stay for those last few attacks before finally melting away into dust.

Imagine if Gaster injected himself with some of Chara's determination. It may have just been out of pure scientific curiosity, but as to why he did so is unimportant. What is important is that he did, and that he left behind information about the ideas that he had and some of the effects that it caused. This information would ultimately be passed down to Alphys, who was able to inject more monsters with the determination of the five other humans that fell before Frisk and after Chara, and created the amalgamates.

Now, it is very obvious that Gaster is also an amalgamate, though maybe not in the sense as the amalgamates are in the True Lab. What really matters is that he, like the other amalgamates, are still here after death. His ultimate demise was the Determination Extraction Machine, which injected him with the determination that inevitably tore his body apart.