Actually, that depends entirely how you read the scriptures of Christianity and of Islam. Why do you think that for most of the 2000 years of Christianity's existence people were killed by authorities with express reference to the Bible? Near my home, new monument just went up memorializing one such event:
325 years ago, a whole bunch of people were killed by devout Christians who'd been taught, in no uncertain terms, one of the faith's commandments:
Thou shalt not suffer a witch to live.
The Bible is full of death sentences for all manner of violations of religious law, from serious to trivial. For example, a child who talked back to his parents was supposed to be taken to the village gates and then everyone in the village was supposed to throw rocks at the kid until he was dead. It's a brutal religion.
But, like all religions, you can also interpret it very differently, by putting focus elsewhere. You can jump through various mental hoops to allow an
a la carte approach to scriptures, and thereby excuse yourself from having to murder people whom the scriptures clearly insist should be murdered. Many Muslims do the same thing.
If you want an eye-opener, take a look at the list of states that impose the death penalty, and then a list of states by level of religiosity. The least religious states in the US (ranked by the percent who told pollsters that religion is "very important" or at least "somewhat important") are VT, NH, MA, ME, WA, CT, MT, OR, AK, and NY. Of those, six out of ten have no death penalty. At the other end, the highest religiosity is in AL, LA, TN, MS, OK, TX, KY, AR, WV, and SC. Of those only one out of ten has no death penalty.
What do you make of that? Why are the secular states six times more likely not to allow killing people who break the law than the religious states? Whatever the answer, it clearly isn't about Islam, since it's an insignificant minority in those death-loving religious states. They all have large Christian majorities, with most being evangelical protestant majorities. In fact, the very large majority of executions in the US, since 1976, have happened in the South, which is the most fervently Christian part of the country.