AITA for not wanting to pay for the full amount. so I borrowed my friends computer yesterday to watch a TV show my friend recommended, I had made myself a coffee and was drinking it and watching the show. 

I told my friend to be careful as I had a coffee and I assumed he took that, he did not as he decided to scare me by leaping behind me and screaming. I got a huge fright and split coffee all over his keyboard. 

he now wants me to pay for it, I've offered half but he thinks I'm being unfair. 

AITA?