Why would he lead me on when he had a whole ass girlfriend?

first and foremost, I want to start off by saying this guy was my best friend. A month ago I asked him if he had a girlfriend , but he told me he had hoes. Fast forward, a few weeks ago he told me he really liked me, and told me I was his girl. He did ask me for pictures, and other things. So, moving on to this past week, I found out he had a girlfriend for two months, and they were turning three months. I might be an asshole for what I did, but I messaged the girl , and I sent her screenshots of everything. I also told her I didn’t know if her existence since he didn’t tell me at all. Right after I found out, deleted his ass off of my socials and contacts list.
But, I don’t know why he would lead me on or play with me. I feel stupid for believing his crap though.

Why would he lead me on when he had a whole ass girlfriend?
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