Board Thread:General Discussion/@comment-10788405-20160720031315/@comment-27701762-20160720032855

Presumably one of the things Asriel could have feared was retaining the incredible amount of power that the human souls would have given him. Importantly, human souls do not innately grant compassion, as is explained (so to speak) in one of the books in the Snowdin Library, and is also evident by the fact that Photoshop Flowey does not suddenly become compassionate.

Also important is that while his experience as Asriel allows him to feel compassion again, it isn't specified that he retains this feeling upon tranforming back into Flowey. In fact, by all evidence Flowey shouldn't retain this feeling itself. What he retains upon turning back into Flowey is the memory of the feeling, which is enough to make him repent of his previous actions.

So ultimately Flowey is likely abandoning the human souls out of fear that he might not be able to stop himself from eventually reverting to his original, sadistic self.

More relevant to the point at hand is that Flowey can only transform into Asriel (i.e. regain his monster form) when he has the combined power of seven human souls. Even with all of the human souls left, he would only have six, which would be enough to turn him into Photoshop Flowey, but not into Asriel. Keeping one or more of the souls would make him more powerful, but wouldn't make him Asriel properly speaking.

It's not quite clear that the human souls "overload" Flowery. All we know is that they "rebel." Flowey refuses to go through the fight again if you've already beaten him, since he knows that the same series of events will play out. But he is willing to absorb the human souls again once you have gathered the monsters together, since he knows he will (at some point) be able to absorb the power of the monsters and free himself. So human and monster souls having "energy" that certain bodies can or can't withstand doesn't quite fit with the narrative.