skygiants

Death doesn't freak out on Mort for no good reason. He's pissed off because Mort is changing history. It's not just the lifetimers that are affected by Mort saving Keli; as Mort discovers, the biographies in the library also contain future histories. If Keli had died then the reign of the evil Duke of Sto Helit would have led to a long period of peace and stability between the city-states of the plains. Now, if Mort had just been willing to let the reality bubble collapse then history would have corrected itself automatically, but he didn't - he used the abilities Death had given him to try and sneak Keli out of the bubble. Basically, Mort's hormones came close to damaging the fabric of the universe and possibly causing a number of wars to boot.

As to why Mort and Death are OK with each other at the end: as a creature who is outside time, Death knows there are two ways for history to be put back on track. One is the original plan - Keli dies, Sto Helit rules. The other is if someone steps into Sto Helit's place and unifies the states anyway despite Keli being alive. Hence Death inviting Mort to challenge him; both of them know that Mort can't win, but by challenging Death Mort shows that he is ready to give his life to keep Keli alive. That lets Death know Mort is willing to spend his remaining days working to repair the damage he's done - something Death cannot know during the duel because as his apprentice Mort, too, exists outside of time.

However, at the wedding Death is slightly cold to Mort because now Mort and Ysabell are within time again, Death knows that the price of letting Keli live is to see his adopted daughter die. Death is ready to accept that because his whole plan in hiring Mort was to marry her off and let her live, but Mort's mistake has also doomed them both to die comparatively young.