Muslim's just following their Islamic beliefs?

The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter;
Qur'an 5:33

Narrated Aisha, Ummul Mu'minin: The Apostle of Allah (peace_be_upon_him) Said: The blood of a Muslim man who testifies that there is no god but Allah and that Muhammad is Allah's Apostle should not lawfully be shed except only for one of three reasons: a man who committed fornication after marriage, in which case he should be stoned; one who goes forth to fight with Allah and His Apostle, in which case he should be killed or crucified or exiled from the land; or one who commits murder for which he is killed.
Abu Dawud 38:4339

Narrated AbuzZinad: When the Apostle of Allah (peace_be_upon_him) cut off (the hands and feet of) those who had stolen his camels and he had their eyes put out by fire (heated nails), Allah reprimanded him on that (action), and Allah, the Exalted, revealed: "The punishment of those who wage war against Allah and His Apostle and strive with might and main for mischief through the land is execution or crucifixion."
Abu Dawud 38:4357

Narrated Abdullah ibn Abbas: The verse "The punishment of those who wage war against Allah and His Apostle, and strive with might and main for mischief through the land is execution, or crucifixion, or the cutting off of hands and feet from opposite side or exile from the land...most merciful" was revealed about polytheists. If any of them repents before they are arrested, it does not prevent from inflicting on him the prescribed punishment which he deserves.