Was Adam as stupid as most horror film characters?

He was so massively STUPID...if it wasn't for TIM ROTH...I would have turned the movie off.......Keeps the cellphone, in the door...dumbrig smile...holds his head when he cannot take it...very slow film.....tough to keep going...

Adam wasn't stupid. He was simply young and naive. Naivety is not the same as stupidity. I found the character quite engaging and likeable. I laughed out loud a number of times at his antics. You've got to remember this teenager found himself in a situation in which he was out of his depth. He was 19 years old and used to slacking all day on computer games. Having been thrust in the middle of a hit gone wrong, I'm mot sure how you would expect a 19 year old to react other than how he did.
Reducing Adam to the level of stupidity seems to me to be a complete misreading of the character.

He's 19. He's young and naive like stated by others, he's pretty much a momma's boy, and he's been living under intimidation from his mom's gangster boyfriend.

He was annoying, spoiled and didn´t isten, true.

But I think that was quite realistic!

Mom gets divorced, finds a rich sugar daddy. New daddy doesn´t want to waste his time with the son => throws money at him to get him out of sight => son becomes like in the movie.

Besides the "moms new lover = gangster & wants to finish the boy off" part, that´s pretty common!

I liked Adam, I think he acted like any young, naive 19 y/o boy of his background would in a situation like that. I mean, come on, give the kid a break...it's not everyday that he get's thrown into the clutches of a hitman 30 years his senior. I think Adam may have found Roy's job rather glamorous too, figuratively speaking, and really wanted to "fit kitchen's" with him...he just had some unfortunate mishaps since he was not exactly trained for Roy's line of work. And as Roy put it, "Son...it's not for everyone." All in all, I think Adam kind of looked up to Roy almost as a father figure in a way and maybe tried too hard to earn his approval, therefore causing him to screw things up even more...accidently of course. I've been in awkward situations when I was Adam's age...I know how he felt...and I know how easy it is to screw things up when you don't know exactly how to do what it is you're trying to do. Adam may have been annoying at times but, to me, it was in a innocent sort of way which made him very likable...and it seemed to me that Roy tapped into that innocence which formed a bond between them. Yes...I think that may have been at least one reason why Roy couldn't kill Adam.

"Ain't life grand"

I think the Adam character was deliberately constructed that way to showcase the enormous contrast in behaviors and life experiences between the two main leads. He is also there to render comic relief from time to time in an otherwise serious script and gruesome subject of murder. If you feel annoyed by Adam's behavior and he comes across as clueless and brain-dead, then O'Connor has done his job well, LOL. Really, how much maturity can you expect from a 19 year lad whose main forte is slacking off.

He was an idiot. 19 going on 12. If you're that thick at 19 there is no hope for you