How can a Muslim combine the fear of a bad end with inner peace?

There is no contradiction between fearing a bad end and God's punishment, and having tranquility. True tranquility comes after fear, because a servant's fear of a bad outcome drives him to seek refuge in God, obey Him, avoid disobeying Him, and frequently remember Him. This in turn brings about tranquility. The Almighty said: "Unquestionably, by the remembrance of Allah hearts are assured," and His saying: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life." God has informed us that the angels reassure His pious servants who said: "Our Lord is Allah, and then remained steadfast," at the time of death and gathering, giving them glad tidings of Paradise. Had it not been for their fear of God, they would not have attained this tranquility.