Looking at something you accidentally stumble upon isn't peeking. Accidentally setting your eyes on something you had no idea was there beforehand is simply "noticing" something. Are you "peeking" if you happen to see someone on the street walk past you when you turn a corner? Your vision just happened to be set on them in that moment. Peeking would be to intentionally spy on someone you know is there, with the intention of looking at them without getting caught. That wasn't the case here. He really did just stumble onto them. He knew she was there because he happened to see her, and the realization of what he was looking at is why he came to the conclusion that he shouldn't peek.
Though it's not like I'm calling you out or anything. We may just have different definitions of what constitutes "peeking", which would mean neither of us are wrong in our own definitions.
I can't believe people believe he's killed with this little, after having survived the super-kick to his face earlier. This punch is at most half as powerful, probably less.