Why does islam say Allah is Merciful when he demands death penalty against gays and lashes for another sins?

and as well as Allah demanded to mohammad a holy war.. it's clear the Quran referrred to the war between mecca and medinah.. during the mohammad's life and he was the leader of this war, and now returning for the horrible punishments like ity is a sin or not.. I don't think muslims couldn't judge that even they say believe in science too.. according to what I have learned from islam even I grew up as a christian. that islam is very strict and don't let other religions in the islamic lands... if they do.. they are citizens of a second cl because they can't preach freely their faith that's another topic, because muslims come to the west and they build beautiful mosques in europe like in london or usa but returning again where is Allah the most merciful? he demands to punish with the death penalty to gay people and don't give free to other faiths specially christianity because many christians are arabs and they live in the middle east.