If they knew before take off that one of the engines wasn't working then they shouldn't have tried to take off. I am unfamiliar with easyjet but If I remember correctly they operate Boeing 737s and A319s. I'm not sure if they can take off with one engine but if they can they would need a heck of a lot of runway to get enough speed to get off the ground. There is no guarantee it would start after take off and attempting a take off would be stupid and probably unsafe. I doubt they had a engine not working or any indication of an engine problem pre flight. Now if they were rolling out and it failed and going too fast then there is no stopping so they would have to continue taking off. They can fly good enough with one engine to get it safely on the ground if an engine failed during flight.