Personally, I'd like it more if he were straight-up evil because if we have too many morally grey backgrounds, it'll get boring. Sometimes you gotta have characters who have few (if any) redeeming qualities, and end up just being complete bad guys. Singed could fit this bill nicely, IMO, as Zaun's major villain.
But that's just my opinion.
Go read Engineering the Nightmare
And you tell me Singed isn't a horrible evil bastard
While they made some villains more grey(and Warwick even became an anti-hero) I doubt that Singed will become Gray. I mean seriously,Singed kidnapped and tortured Warwick pretty much just "for science".
And riot has shown already that they are fine with making morally black champions I mean 
and 
are nothing but evil(and they were like this already when they were alive)
The torture was a side-effect though. Much like previous subjects dying. But yes, full on sociopath/psychopath. Doesn't mean he can't be an interesting character or likable. And the recent Viktor fan service seems to indicate that Riot is capable of showing both. So thats nice.
Evil. He's always been in the pitch black on the moral scale
Do you know how Singed is played?
Singed doesn't give a flying fuck about anything, teammates or morals alike. He just does what he wants.
I hope Singed becomes a villainous character in Zaun. Not gonna lie, I was slightly dissapointed by Warwick turning into an Anti-Hero, but reading his lore it pretty much makes sense. I also hope they keep 
evil and not morally gray.
I've noticed that basically everyone in Zaun is morally grey or somewhere in between Neutral Good and Chaotic Good. If Singed is Evil, he will be our first truly evil person from Zaun. (Twitch is an animal that can talk, that would be like calling a wild wolf evil for killing someone to get food)
Think of Singed as the Viktor of chem-tech. If Viktor somehow servives for another century he would be a morally black character pushing his ideals of the Glorious Evolution onto all. This is probably where Singed is at in current time (he is over a century old btw) choosing to experiment and push the boundaries of chem-tech, which may be for selfish/cruel reasons now, but every mad scientist has a humble beginning. Although, this means nothing now because we know that he has killed 1087 persons before he 'succeeded' on Warwick. And there is a possibility that this was 1087 persons killed from this specific experiment, and he has killed hundreds of thousands more, on other experiments. So my guess is that he will be morally black, but with a vague, distant, irrelevant grey past. Inb4 Singed created Urgot.
Is Singed evil? Or does he just do what he must because he can? Singed isn't Morally grey, his morals simply don't exist. As far as Singed is concerned there's no price to steep and no blood to precious for triumph in the name of science. I don't think Singed has ever been not a bad guy. Especially since he took over Warwicks bit in the lore. He turned people into piles of goop not even because someone paid him too, but, as Engineering the Nightmare pointed out, in the name of scientific discovery.
As far as being The Villain I don't know. Singed seems fairly self centered in his experiments, and so unless he makes you involved it's unlikely you will be. For a champion to be The Villain they would have to be antagonistic to nearly every other champion in the city state. However Singed only has relations to two other Zaunites according to his Universe page.
