After months of lying to his superiors to prevent Nabu knowing about his animosity… he shoves his animosity in Nabu's face for no apparent gain. While he still has four months to go before he is ready.
Why?
Because he does NOT want to have to fight BOTH Nabu and the entire League. He wants the League on his side, or at the very least neutral when the fight happens. For that to happen he need to hit them on the head with the "Nabu is a bad guy" Clue Bat hard enough that is sinks in.
Remember he had actually said to Wonder Woman (paraphrasing here)
"If you are actually going to recruit a supervillian into the Justice League, Circe is a better choice then Nabu.
"
Where Circe is one of Wonder Woman's Rogues Gallery of villains. Instead of actually considering that, and the Bad Stuff (TM) Nabu had
quite recently done at that point in time, which she was
also complicit in, she decides to treat what OL said as a joke and induct Nabu into the League
while he is wearing her friends stolen body as a suit, rather then face some inconvenient truths about what the whole affair says about her as a person.
So obviously, that previous Clue Bat impact setting and frequency was insufficient. So he readjusted his grip on the old Clue Bat, and tried again.
Also, remember that this whole situation can be traced back to a betrayal of Nabu by the Justice Society, of which Wonder Woman was also a founding member. From what we have been told in the story so far, Nabu and [can't be bothered to look of the dead guys name] were partners as Dr. Fate (Dum dum dum), but Nabu was an ass hole in that relationship, and to was implied he tried to sabotage Mr. Nelsons [think that was the name] personal relationships so he would have less social contacts, and less reason to be anything other than Dr. Fate. Yes, this is incredibly fucked up. No, all bad deeds do not merit the same sentence, and 50 years of solitary confinement is way out of proportion to the crime.
Now, Mr. Nelson was able to find someone and get hitched anyway. Good for him! And his new wife convinced him to just, NOT be Dr. Fate anymore. OK so far. Apparently that somehow translated into "Put him on a shelf for 50 years unless I feel like reliving old times for an evening every few years or so."
So now we get to talk about Superhero Racism and Fair Weather Friends. Because for Nabu to be treated this way required the rest of the Justice Society to be complicit in their silence. These were his Team members, and really should have been looking out for him. Instead Nelson locks him in the house for years at a time and does not let him leave, and everyone else goes "I'm sure everything's OK." Because they did not really see Nabu as an actual person, but more like an accessory to be worn by the people who actually matter, the homo sapien sapiens club, AI don't count, nor do possessed helmets.
And this is still going on in the present day. It seems like Nabu let Nelson treat him that way because they had an agreement, and he was horribly, horribly mistreated. So for the next host, he decided not to actually make an agreement he would be Orderly Bound to, just take. The deal was sealed when the "New" Justice Society, the League said they would find him a new teammate, and instead Wally
put him in the Trophy Room. So when Zatarra puts him on now to discuss hosts, but because the shit has hit the fan,
again, he notes she has not Idea about finding him a host. At this point,
Fuck them all. Treat them like the betrayers and Oathbreakers they are. No deals that benefit them over me, no more Mr. Nice Guy.
And Nabu is a villain. And to play devils advocate here, Zatana was the one who said to the team and Paul (in regards to the host situation, and again paraphrasing here) "This decision will be decided by the League, leave it to us" So Zatana was the one who 1: explicitly took responsibility for the matter on behalf of the League, and 2: denied anyone else involved input or any ability to help.
And then he put on Nabu's helmet, and Nabu could presumably see exactly how much effort they had (or had
not) put into the matter.
TLDR, Nabu is a Villain because the JL and it's predecessor the JS was institutionally racist against people they did not consider human (like him), betrayed him as a fellow member and teammate, and basically broke their obligations to him at every opportunity. This conga line was Nabu's Villain Origin Story, instigated by the "Defenders of Law & Order" and they burned his trust in EVERYONE. Paul works for the JL, who he cannot afford to trust, and ALSO worships a goddess of discord. He gets no trust extended to him, because all the other superheroes took ALL THE TRUST, poured gasoline on it, and set it on fire.
So Nabu thinks he NEEDS a human body to actually be treated as a person, and this is all a trap by people who have broken faith with him before. Of special note is Wonder Woman, who was there when this all began, and did absolutely nothing to help. She has a lot invested in not thinking about it critically. So she does not, and instead votes Nabu onto the League, because it is easier than owning up to her part in the matter.
Chill, guys. We can argue about the importance of a small portion of the worlds moral views later. 
it's not a binary, 'Either or' equation. By that logic, the current tech emphasis should make magic, including that of Nabu, The Zataras, Klarion, and Circe, completely ineffective.
