Karna's useless war

When someone go to war,he should slay the enemies.Thats what war is all about.Karna giving a boon to kunti that he will spare four pandavas except Arjuna made no sense.He defeated all the pandavas except Arjuna ,and spared them.He almost killed Arjuna with his nagastra but Krishan saved Arjuna.When his chariot wheel got stuck,shalya didnt help him.Its useless to give such boons to anyone when someone is going to war.It makes one to be defeated in the war.And also,victory which would have been won,becomes a self defeat.And karna didnt get credit for sparing the lives of pandavas later.The pro pandava writers criticised him for hate against  Arjuna as he didnt forgive Arjuna,and also for insulting other pandavas.He forgave four pandavas,but thats not enough for them.And the pro pandava writers manupulated facts,that he was really spared by the pandavas before he spared them.And he abused them,and deserved to be killed.
So his boon to kunti that he will spare fur pandavas didn't benefit him.And that made him to be killed in the war.So karna shouldn't not give those boons.he should have rejected kunti,and said he will fight for on the kaurava side.Or he should have accepted kunti and joined pandavas.These kind of boons are useless.
Also Arjuna is adharmic warrior and killed karna when his chariot wheel was stuck in earth,and didn't wait till karna lifted it.And Arjuna is praised for his adarmic way of killing people.karna had defeated everyone,the pandavas,satyaki,drishtadyumna.But this boon didn't make him win the war.