I wonder that too, the trailer seems to imply that there's an element of tragedy that gives rise to Electro. Which makes me wonder how he went from a Spider-Man fan to a villain who wants him gone (maybe Spider-Man's responsible for Electro's birth, unless comic-con said otherwise)?
i think the simple fact that spider-man engages him at times square will send him over the edge. he may identify himself as max and spider-man replies "who?", which makes max snap.
from set pics and videos from earlier in the year, max is wandering around times square in pain, which likely eventually leads to an accidental expenditure of energy that makes everyone freak the hell out. the cops arrive, but to no avail. max is like a wounded animal at this point. how this accident occurs is not clear, but in the comic con footage you can see max wander in front of an oncoming bus.
all the commotion leads to spider-man arriving on the scene. max realises that he is there to stop him, not to help him, and this betrayal is the straw that breaks the camel's back. electro is born. and he's pissed.
the success of electro will depend greatly on painting max as sympathetic prior to his accident, and making spider-man's perceived betrayal something audiences can empathise with. also, they'll have to be careful not to make spider-man come across as the bad guy when meeting electro at times square for the first time. if he forgets who max is, hopefully it is justified, and not simply because spidey thinks he's the cat's pyjamas (i think peter is genuine when he saves max on the street, he just has a lot on his plate so naturally forgets who he is by the time times square happens - plus he looks different haha)
of course, spider-man might remember max, but nevertheless try and aid local authorities, which is seen as a betrayal by the former.