99.9 percent sure he's a cheater

Not posting my name because of the fact that I'd hate to give someone away.

But I started talking to this guy and he's super sweet but pretty sure he's a major cheater and an active cheater. Here's the situation.

He claims he's on speaking terms with his ex which is okay they have 2 kids together kinda gotta talk to each other well my best friend guy but he's way outta my league but positively my best friend,love him,to pieces we study together because we are both seeking the same,degree but the best friend doesn't hit my radar. Well since active cheater and I started talking her ask where i was and it seemed to always be with the best friend. So the best friend and I backed off. Everything was,going good with this guy that I think is a,cheater. Like great now no best friend issue. Well last week I got stabbed (long story short it was a battle injury while running) well someone at bumped into me at the university while I was walking out side with said best friend. It hurt so I break down crying and my best friend is just like calm,down and hugs me well said active cheater pulls up and ask who the handsome b*stard hugging me is. I freak out and pull away then explain I got bumped in the stab wound and give active. Cheater a kiss he claimed he understood and knew my best friend was harmless and it was just well that's what friends do when your friend is crying but now his ex is all over his timeline he hasn't,spoke to me in days since the accident and shes all together since 2010💝 and I love you (name here).

So real question did I do something wrong or was he playing,the field the entire time or did best friend have something to do with it?

Please note best friend is 15 years older than myself, so dating is majorly out of the question we just have the same major and study together and have for a long time because its mutual benefit having someone to work with on graduate classes. The studying has been going on for over a year

Also note cheater is 4 years older than myself with a degree in what im graduating in this December. We started talking 2 months ago.

Final detail I'm 21 so theirs the math for you all and a double major so not like I'm running,around drinking and partying I'm focused on school.

Final note seriously. Yes ive been over to best friends house before yes i,never think when cheater would ask where I was or what I was doing I believe honesty is the best policy and yes cheater new best friend was my study buddy i,told him up,front when we started talking.

What's your take please help, yes I'm reading the comments.