I haven't seen the film yet. It doesn't open in Chicago until 9/24 so I can't offer an opinion until then.
I saw the show initially at Second Stage in spring, 2016. I think the plot device at the beginning is very contrived, so much so that it strains credibility. The story itself is highly manipulative. Nevertheless, I was blown away by Platt's performance and it struck a chord with me. I saw it again a year later on Broadway and was equally moved.
Apparently there are different interpretations of Evan's character and actions, especially on ATC. I took him to be a troubled teen who suffers from anxiety and depression and has difficulty connecting to his peers. When Connor's parents confront him with the letter, he tells them the truth but Connor's mother won't accept it. Evan isn't strong and mature enough to make her see the reality of the situation so he let's her believe what she wants to believe. His decision, of course, is morally wrong and the story that follows leads him into a web of lies and deception.
I see Evan as basically sympathetic. Others see him as Machiavellian. The stage version of the story worked for me, but obviously not for others. I'm actually a little surprised that it was made into a film. However, I'll wait until I see it in 2 weeks before commenting about it.