The Sahih of Imam al-Bukhari (#6862) has the following hadith from `Abd Allah ibn `Umar (Allah be pleased with them both):
لا يزال المؤمن في فسحة من دينه ، ما لم يصب دما حراما
“There will remain forgiveness for the believer in his religion except if blood is shed unlawfully” (ma lam yusab daman haraman).
Severity of spilling Muslim blood, i.e. it is sacred, protected.
Deliberate killing of a believer could be an act that can compromises the integrity of one’s belief (iman).
Allah knows best.