That’s respectable. I feel like there’s a few possibilities, one was he actually messaged you quite a few times (and chat was so messy you missed most of them) and got salty, or he’s just an unpleasant person / had a bad day.
Either possibility is a loss for him; you’re definitely not the only person who doesn’t respond immediately to trades. If he pm’d threats to everybody that didn’t answer him that’s just a waste of effort. I usually move on if someone doesn’t respond because there’s just too many reasons for me to angry about (they could be afk, doing something, trading in USW2, too lazy to answer / take down effort etc.)
Actually I’m quite curious about the scammer flagging system now, and if it takes into any other factors other than number of reports, such as credibility of the reporter (perhaps based on age of account, star rank idk). I think @Scorchmist wanted to test this out in the past with alts and mules, I’d be happy to volunteer as tribute but I digress.