Is it wrong to talk to a guy in a relationship?

There’s this guy I went to school with, we’ve both left but he’s a real player. I use to talk to him before he got a girlfriend and he offered to take me on dates and said we could be whatever I wanted but I wasn’t into him because I just knew he was a player. He got a girlfriend quickly after that and we still spoke from time to time a year after that. I do really get along with him we have a lot in common and I find him really, really hot. But I know he’s a total douche and not loyal at all.

So I just got out of a relationship recently, literally a few weeks ago and I’m a mess atm, I’m not over my ex at all and am not interested in anyone. Weirdly he starts messaging me and “checking up” on me but also starts flirting with me and he lied about not being with his gf anymore. I’m feeling **** about myself because of the way my ex left me so I did flirt with him and I know that’s no excuse, I’ll never ever meet up with him or anything. He wants to meet up with me and I’ve just kind of been leading him on because I’m just enjoying the attention since I no longer have my ex to talk to 24/7 anymore but I’m not interested in him and would never ever meet up with him. Also, he’s the one that keeps reaching out to me. Another really awkward thing is his girlfriend lives on my street and I’ve seen her a bunch of times! I know I need to stop replying to him and I’ll be going back to uni soon it’s just I am at the lowest time of my life but I will get out of this. Thing is I’ve never met him before in person, just passed him a few times in school. He started following me on insta after we left school and had no idea who I was so I’ve never even interacted with him in person.