Lister probably got bitter after seeing Sylhan with another man. He probably thought he belonged to him or something being the stuck up bastard he is
It’s an inferiority complex/one sided rivalry combined with a crush. Lister and Sylhan were friends since they were kids, but Lister always felt interior or maybe jealous. And he had feelings for Sylhan, but probably let his inferiority complex get in the way. Then he found out Sylhan loves someone else and that probably spiked his inferiority complex and also probably pissed him off cause he felt like Sylhan was his first. And now the Sylhan is a slave and forgot his memories, Lister feels this false superiority over Sylhan now, especially cause he thinks he “owns” Sylhan. Thats my take on his character.
I forgot to mention, Sylhan loving Gjord is also probably why Lister hates Gjord. Lister is just a bitter, jealous guy at this point. And kind of a tragic character too.
ok I'm on like ch 10 and I don't understand the relationship between lister and sylhan like it shows flashbacks of them playing together and clearly being friends but in present time lister treats sylhan like shit and sometimes lister will act nice and even treat sylhan how you would treat in terms of like how friends can be sarcastic with eachother and stuff and clearly have a more casual relationship than any other master and servant I just don't get it bro