The difference between a believer and a Muslim

I had a misunderstanding of some verses of the Quran. In 4:93 it is said that whoever kills a believer will be severely punished. > And whoever kills a believer intentionally, their reward will be > Hell—where they will stay indefinitely. Allah will be displeased with > them, condemn them, and will prepare for them a tremendous > punishmen.4:93 But we know that according to 49:14, not every Muslim is a believer. > ˹Some of˺ the nomadic Arabs say, “We believe.” Say, ˹O Prophet,˺ “You > have not believed. But say, ‘We have submitted,’ for faith has not yet > entered your hearts. But if you obey Allah and His Messenger > ˹wholeheartedly˺, He will not discount anything from ˹the reward of˺ > your deeds. Allah is truly All-Forgiving, Most Merciful.”49:14 I even read that it takes a lot of effort to achieve the degree of a believer, because faith must enter the heart. Therefore, my question is: why is the word believer used in 4:93, and not Muslim? Does it make sense?