[47.3] That is because those who disbelieve follow falsehood, and have given them their dowries, taking (them) in marriage, not fornicating nor taking them for paramours in secret; and whoever denies faith, his work indeed is of no account, and in the hereafter he shall be one of the losers.
[47.4] So when you meet in battle those who disbelieve, then smite the necks until when you have overcome them, then make (them) prisoners, and afterwards either set them free as a favor or let them ransom (themselves) until the war terminates.
Note how Muhammad says that Muslims should kill non-Muslims because they are non-believers and therefore follow falsehood. The other reasons given in [47.3] are unclear. Why after saying that non-believers are doomed to eternity in hell does Muhammad order his followers to kill them? Probably he was trying to motivate his followers to do battle with the non-believers so he could extend his power.
[47.35] And be not slack so as to cry for peace and you have the upper hand,
[48.29] Muhammad is the Apostle
of Allah, and those with him are firm of heart against the unbelievers...