No, I didn't think it was wierd. I think the episode showed that he felt really bad. When you're at the point that you want to kill yourself I don't think you're thinking rational, so maybe he took the rifle because that was what he had. The place might've been a way for him to get people to notice him, which was part of the reason he was feeling bad - he felt invisible. That can really, really hurt when you're a teenager (and a adult!), especially when it happens for a very long time, and it does lead to suicide.
I agree with [MENTION=10438]JCoatsworth[/MENTION] that when you later got to know Jonathan he didn't come across as a person that would kill himself, but then again: very few people does. There isn't characteristics or a specific personality type, there are so many reasons people feel like they've lost the will to live. And it is possible Jonathan grew from the experience and had become different at the point we learnt more about him.