At this point, it's impossible to tell if Ezra really is "A", or if there's another twist coming our way. Let's look at the reasons he could be "A" — and the reasons Ezra fans can legitimately hold on to some hope.
Why Ezra Probably Is "A"
They can't pull a Toby again, can they? The finale was clearly suggesting Ezra is "A": He showed up at the lair in black, he looked super pissed, and the promo for the Halloween episode even used the #EzrA hashtag. Yes, this could all be a big mislead. Ezra could just have been following Aria, or he could be part of the "A" team to protect her, or because of blackmail or something like that. But the writers already went down that path once with Toby and his non-betrAyal. They wouldn't back out of a big twist like this again, would they? That would make them the Writers Who Cried "A" — we'd have a hard time ever trusting them again.
"A" never really messed with Ezria, despite targeting every other pretty little relationship. This always frustrated us. It made no sense that "A" would go out of his way to do things like create an overly complicated scheme to frame Caleb, and yet he never targeted the relationship that was actually against the rules. Ezra being "A" solves this mystery.
He owns a typewriter. And we all know "A" uses a typewriter.
He has the money. We've always noted that "A" needs a ton of cash to pull off his crazy schemes, and Ezra comes from a rich family.
He showed up at convenient times. The most obvious example of this was when Ezra mysteriously appeared on the Halloween train just when Aria needed comfort. We found that highly suspicious at the time, and now it all fits into place.
The fact of the matter is, Ezra being "A" would be a brilliant twist, and there are more than enough clues to make it satisfying.
Why Ezra Might Not Be "A"
The "reveal" was not definitive. While we'll be frustrated if the writers backtrack, they certainly left themselves lots of wiggle room on this "reveal." Ezra easily could have been in "A"'s lair for non-nefarious reasons (such as trying to find Aria). Some fans are even speculating that the lair is Ezra's, but that he's not "A" — he's some sort of anti-"A" spy who is using the lair to protect Aria. While that seems more out there, it's not impossible, and the very fact that so much is still open to interpretation means we can't take the most obvious answer as a given.
Would the writers really destroy Ezria? Showrunner Marlene King has always said she's a huge fan of Aria and Ezra's relationship. Would she really make Ezra evil? Of course, if Aria is in on it with him, they could still be an epic, awesome relationship. Just a very, very twisted one. But isn't that even more fun?