Who is at fault? Please read on!

In classtime this guy who I have a really bad relationship with kicked me so I pushed him and from there, it became a full blown fight. Then he made a rule that if person A holds down person B for five seconds, person A wins and same the other way around.

So he was being really aggressive and pulling my leg, stopping me from moving and was kicking my legs but I was taller and stronger so I managed to push him to the floor. Then I got in top of him, counting to five but while he was struggling to get up again, his glass snapped. The right arm had snapped off and he started crying and complaining that I would have to pay a huge amount for new ones.

Is it my fault or his? Should I pay for all of it or just half or none at all? Please help 😭 i can afford it but it's a really big amount and I kind of think that it wasn't my fault and I shouldn't have to pay.