In a scene prior to that, Alphonse found a bug in his phone. I could be wrong, but I think he sent a message asking to meet at 9, figuring that whoever received the message was the one spying on him. If it was one of his crew, they would show up.
Even though Viktor walks through the door, Alphonse doesn't believe it's him for a couple of reasons:
1.) At the beginning, Viktor saves Alphonse's life when he could just as easily let him die or kill him himself. Therefore, Alphonse trusts him above the rest.
2.) Whoever is messing with him has already tricked him into killing the wrong person once. Now maybe he's trying to trick him into killing people on his own crew to further isolate him. Whoever bugged the phone could have just relayed the message to Viktor so he'd fall into the trap.
Or the actual reason. He was asked to come and bring food.
Remember victor says "heres your food". And al says "what did you tell darcy?" He replies with "I just told him I had something to do as you said dont tell anyone".
But Alphonse also seemed surprised to see Viktor, and said something along the lines of "I figured whoever showed up tonight was the one", like he wasn't sure which of them it would be.
I don't know, but it seemed like he found the chip in his phone and, using the techniques of Jessie Slaughter's father, backtraced it, and sent the mole a message to meet up so could tell him "YOU DUN GOOF'D!" face to face. Or not :/
I totally misinterpreted that text, that makes sense thanks. I recall Victor's cell vibrating when he was with Darcy at the cemetery and he very briefly looked at it (it didn't show the screen), I didn't think for a moment that the text was from Alphonse.
This was the only part of the movie that left me confused because while it totally looked like Victor walked right into the trap, it didn't make any sense; Why would he show up at 9 if he wasn't invited.
I see a few explantions about the scene, and it still makes no sense to me. It seems he had him red-handed. Still don't get it. So if anyone would like to explain it to my dumb @$$ in laymen terms, then please do.
I don't think anyone else explained this correctly. Alphonse however, perfectly explained his response to Victor showing up at 9pm.
First, Alphonse found a bug in his phone. So he knew that whomever was sending the letters and pictures was probably the one who put in the bug. He then puts the bug back in his phone and makes a phone call to an associate telling him that he will be meeting with a lawyer in this building at 9pm. The associate says, "what if you know who shows up and does something crazy." Alphonse responds that it won't happen because no one knows about the meeting except for Alphonse, the lawyer and the associate on the phone. He then sets up the trap.
Victor gets a text from Alphonse to meet him at the building and not to tell anyone about it. This is why Alphonse asks Victor what he told Darcy and Victor said that the text said not to tell anyone so he just said he had business. Not sure how so many missed this. Alphonse sent the text and asked Victor to meet him there. He then says that he made it clear that whoever showed up at 9pm was going to have a bad night. Since Victor showed up, he figured that he wasn't the stalker because why else would he have been crazy enough to show up. Alphonse goes on to explain that Victor is the only one he can trust because he saved his life in the shootout earlier. He then goes on to basically say that wouldn't it be crazy if Victor only saved his life because he wanted to be the one to kill him (him meaning Alphonse).