Character deaths always belong to the most unpleasant moments in Fire Emblem. Though how to deal with them? Resetting or continuing, as if nothing happened. From my point of view I usually don't restart a chapter, because it's part of the challenge in Fire Emblem. If I did a mistake, I have to suffer the consequences. The exceptions are:
if I need the unit later for an important recruitment or a special conversation
if I couldn't prevent the death, because the RNG screwed me (f. e. I can't recruit a character in time)
if I play on very high difficulties (FE6 HM or FE11and FE12 in H3 or higher), because I need as many good units as possible
I'd even sacrifice an unit, if I have no other choice to prevent a death. If someone is in acute danger, I put a weaker unit in an attacking range of the enemy to "rescue" the better unit.