Yes. Because they deserved it. Now if you don’t know what the amorites were doing you will think God was unjust but he was not.
Now God sends the Israelites to Egypt for 400 yrs because it wasn’t yet justifiable to kick the amorites out of Canaan according to what he tells Abraham in genesis 15. It took 400 years for the sins of the amorites to reach the level where they deserved to be punished .
God had to make a treaty with the Israelites which if they didn’t keep by emulating the wickedness of the amorites he would send someone else to do exactly what they they did to the amorites to them. For this purpose at the same time he brought the Israelites from Egypt to Canaan he also brought in the philistines from crete to gaza(you know the Palestinians) who could be used anytime the Israelites broke the covenant as seen in judges, Kings and Samuel.
Some of the things the amorites were doing at the time included human sacrifice, sexual immorality(homosexuality) ,bestiary,inequality, public sex as a form of worship, sex with not only minors but even newborns as worship practice , incest and all forms of corruption.
This are the things Israelites were warned never to do lest they face the same consequences. We know they did this things later on in fact several times and in any case they would be attacked and butchered and taken to captivity in the extreme case. The most recent incident being during the second World War.
During the second World War we know what Hitler did and what was done to him.
During the heat of the war when Britain had been heavily battered Winston Churchill was still encouraged and believed he was going to win the war. One night he opens the Bible and reads in deuteronomy and realise that the only reason he was to win against Hitler just like the Jews story in the book of deuteronomy is because of the evil Hitler had done made him he deserved being killed and loosing the battle.
Hey it may seem odd to some but God will punish those who deserve it. He will either do it personally or through natural calamity like draughts, lightening and hailstones, or he can send bees(hornets). He can also use human agencies. We know he can even make his enemies make dumb decisions and listen to wrong council… So that they can receive their punishment in full.
Next time a tyrant is overthrow and his whole family killed don’t ask why it is happening… And those asking why we still have dictators and evil people around remember it is until their wickedness is full that punishment will come over them and for some it can take upto 400yrs…
Is it right for Christian to serve in the military? Yes and if they kill?