At the end of 13 Reasons Why, we see an anonymous male teenager being rushed to the hospital. It's later revealed that the person in the ambulance is actually Alex (Miles Heizer), who was in critical condition following a self-inflicted gunshot to the head. In the same episode, we see Tyler (Devin Druid) loading up a trunk filled with weapons and ammunition and hanging up photos of the students who bullied him, making some fans speculate that Tyler was planning a school shooting with the photos as his hit list.
However, there is an alternate theory: at the end of the episode, Tyler takes the photo of Alex down after remembering a time Alex was kind to him. Some saw the scene as Tyler choosing to "spare" Alex, whom he doesn't yet know tried to take his own life. Other people, however, saw it as Tyler "crossing off" a name on his hit list — because Tyler already killed Alex. (Or, at least, tried to.)
It's a compelling theory, but one I don't personally buy — frankly, it just doesn't seem like the show we're in. However, Buzzfeed now notes that there's a reason we should believe that Alex really did pull the trigger: he cleaned his room before his suicide attempt. Hannah (Katherine Langford) did the exact same thing on the series before she ended her own life, and that, coupled with Alex's increasingly erratic behaviour, suggests Alex was exhibiting signs he was planning his suicide.
When asked about the theory that Tyler was the one who shot Alex, Devin Druid told Refinery29:
"People immediately started jumping on that theory, and it's so interesting because I never thought about that. Exactly what's on screen is written in the script. I don't know if Brian [Yorkey, the show's creator] has any secrets, or if this is supposed to be a plot element that's up for speculation, or that will be expanded on if we were to go back, but I love to see the fan interactions. I love Miles [Heizer, who plays Alex], I'd hate to be the guy who shot him!"
Looks like we'll have to wait until season 2 (should there be one, of course) in order to find out which truth the series is operating under.