he doesnt sentence him to death; he saves from being sentenced to death.
He did save him when he was about to be shot but later he sentenced him to death again because he was given a fake note according to which Tintin was supposedly a spy plotting against him.
Is Herge saying that it doesn't matter to these people who rules them - they're going to be living a life of poverty no matter what?
From an optimistic point of view, perhaps Alcazar eventually does proceed to improve the living conditions of these people some time afterwards. Besides, Alcazar refers to Tapioca as a bloodthirsty tyrant in "Tintin and the Picaros" and he is critical ofTapioca's justice system while they are watching Castafiore's and the Thompson's trial. Of course this doesn't necessarily mean that he truly is any better and he did have chances to correct things the previous times when he was in charge but hopefully the time he spent as a guerilla made him see some sense.
If Tapioca had saved Tintin from the execution and everything went backwards, it wouldn't have changed that much, right?
Alcazar and Tapioca may not be very different when it comes to their administration and managing approaches but overall, I consider Alcazar a much more OK fellow than Tapioca. At least Alcazar keeps his promises whereas Tapioca does not (he did give his word that Tintin, Haddock and Calculus would be safe if they met him to talk things over and all the time, with the help of Sponz, he was planning to get them killed and make it look like an accident). Which is why I don't really think that Tintin would have been friends with Tapioca even if things had turned out differently. Plus, Alcazar is loyal to his friends and even risked a lot to come over and help Tintin and company escape when he was told they were in danger.
Yes, I'm not overly sure if Tintin likes Alcazar, just puts up with him. I think that that is what Herge is trying to show. Sometimes you need to make alliances with bad(maybe not the right word) people to do good?
I believe that Tintin actually finds Alcazar more or less OK and he tends to refer to him as a "friend" in the series. After all he even went backstage to say "hello" to Alcazar in "The Seven Crystal Balls" ( and oddly enough they both seemed to have forgotten that they had not parted on very friendly terms in "The Broken Ear"). Tintin also seemed glad to meet him in "The Red Sea Sharks" and was asking Alcazar where they can find him etc. Alcazar also refers to Tintin as a friend in every episode that features him.