I'm not sure that Neelix deserves better. He wouldn't know what to do with 'better.' He'd call Kes pet names, sure enough and he would talk to her gentle and sweet, but come a Man with an eye for beauty and Neelix flies into a blind rage fueled by jealousy. Some jealousy is occasionally cute, I suppose, if she knew he didn't mean it, really. Like if he "accused" her of getting it on with The Doctor, since she's been spending so much time with him, or something like that. Just to use that as a way to remind her that he, Neelix, is still interested. But to fly off the handle when a better looking Man eyeballs her ... no good can come from that. He was afraid that Kes would leave him, probably, because he felt she had reason to.
Even in fictional terms, if a woman stays with a Man, it's only because she so chooses. At any time she can just get up and go, never to return, for every reason ... for no reason. And guilting her back would gain him nothing but deep resentment and a repeat performance. In "Warlords," I was so surprised that Neelix did not try to guilt Kes, or beg Kes to stay, that it leapt me out of the story, honestly. I just didn't believe that was him. Then again, they'd been together for a couple years, by this point ... maybe, he felt in his heart, that this was a long time coming.
Maybe he realised that being with her brought out the worst in him. It made him not like himself, very much, because he knew he was never really natural around her. He was never 'himself,' as it were. He was either kissing her arse ... or acting like a jealous teenager because of her. And Kes' staying with him so long, with a Life so short as hers ... maybe she did feel guilty at the prospect of leaving him, because she owed him her Life. Either way, their relationship was never on an equal footing and neither was getting anything out of it that they couldn't have easily found elsewhere.