him

Whenever we see Dennis faced with the idea of killing, he's fascinated/excited, but every time it's been closer to a reality, he's fallen apart/broken down. Glenn himself has said many times (in interviews and on TASP) that he doesn't believe Dennis is a serial killer (or a psychopath, for that matter).

Dennis Reynolds Making a Murderer is kind of directly addressing this by having Mac and Charlie frame Dennis as this cold-hearted killer using shoddy evidence and straight-up lies to prove that Dennis killed Maureen. These lies are "believable" to the audience almost exclusively by the fact that Dennis is indifferent/shows no emotion toward his ex-wife's death. The end of the episode where Mac and Charlie talk about how they can further manipulate his actions to frame it like he's a murderer cuts to show that Dennis is annoyed and also kind of broken? by the fact that they want to see him that way:

Despite all the evidence to the contrary, Mac and Charlie (i.e. the audience) still insist on seeing Dennis as a murderer and will continue to try and twist anything he does to be seen that way.

(And considering they did write and film evidence that Maureen fell off the roof while all alone, Dennis is basically the only member of the Gang you can't connect a direct death to.)

I think at least among fans here, when most people say he's not a (serial) killer they're saying it as in what's rooted in canon: Dennis isn't strong enough to kill anyone. Which might come off as woobification to say he's much weaker than the rest of the Gang, but it's not so much said to excuse him as not a terrible person but said to show how his whole personality is built around fake personas and masking his true emotions to get what he wants (and avoid his true self). Unlike the rest of the Gang, who have a much easier ability to just demand and act with little-to-no remorse, Dennis has to heavily manipulate someone to take advantage of them. He has to convince and be convinced himself that what he's doing is what the other person wants (or believes they want)—which weirdly almost completely rules out the ability to kill (but makes him much worse and more calculated in many other irredeemable acts).