According to Dhaos, half of the cut was clean, right? So if they cut partly through his neck and...hung him up and chained Alisha's body to his lower half or something like that, the weight of it could have...pulled him apart....

[Ugh. There are very few things that make Undyne squeamish, but she's definitely looking horrified at the idea.] Oh, I'm gonna rip the murderer's head off and feed it to them. Let's see how much they like it!