Connor had been suspicious on How to Get Away with Murder since the night Wes was killed and the fire broke out. He was secretive and cagey, constantly starting arguments with everyone and always trying to pin the blame for Wes' death on other characters. Some fans began to wonder if Connor could have been the culprit after all, especially considering that he never particularly liked Wes and didn't seem sorry at all to see Wes go. However, a flash-forward in "It's War" proved the opposite may have been true: Connor tried to administer CPR to Wes before the fire. So was Connor trying to save Wes on How to Get Away with Murder?
The flash-forward was only a few seconds long, so it provided little context for Connor's actions. He was shown giving Wes CPR but without seeing what led to that, it was impossible to know just how deep Connor's involvement went that night. All of Connor's suspicious behavior led Oliver to do some digging, which was when he found out that Connor's timeline during the fire was incredibly sketchy. Connor left Thomas' apartment after their fling with hours to spare before the fire started, but he also showed up at the hospital well after everyone else had already arrived. Connor spent at least part of that unknown block of time at Annalise's house with Wes, so if he didn't kill Wes then there was a good chance he knew who did.
If Connor was responsible in some way for Wes' death, then it probably wasn't a premeditated murder. Connor had always been the shakiest when it came to dealing with the misdeeds the Keating 5 got themselves caught up in, so he probably wouldn't have been rushing to kill anyone. And if he did have a hand in what happened to Wes, then he must have regretted it because he was trying to revive him. Still, Connor killing Wes seems both a little farfetched and too obvious. The show is definitely saving its biggest twists for next week's finale.
If Connor wasn't responsible for what happened, then either he saw what went down or he arrived just after. It was possible that he didn't see anything; perhaps he just found Wes' body and rushed in vain to try and save him. If he did see something, though, then he could be the person who ends up cracking this case. Either way, he was likely the person Laurel saw fleeing through the storm door.
The extent of Connor's involvement might be far from clear, but there's no denying that he knows a lot more than he's letting on.