a climb would have been the best instruction.
How do you know? you have seen the traffic picture the controller had ? Possibly other known traffic in vicinity, or above ? remember what a controller sees is what the radar returns gives him , delayed info..
Capt Hindsight is always right...but better be left alone after an accident.
I also feel very sorry for the controller. I hope they do have CISM in place in his unit, because if he/she feels responsible for the collision he/she most probably will never actively control again.