Indeed lolA person's form should not affect your decision.
In which case that's an indirect free kick and you can't give a red card because it's not one of the seven direct free kick offences committed with excessive force (thus not 'serious foul play').It came under playing in a dangerous manner.
After the game I was chatting to several blue players and they said we thought you had a good game but you should have sent that boy off. Explained why I had gone just caution and they accepted it.