Some classic movie bad guys have great motivations, like Darth Vader ("corrupted by the Force"), Voldemort ("I don't have a nose") or the shark from Jaws ("I am a shark"). Others are just crazy, like the Joker.
But in a lot of cases, the bad guy is just an excuse for the hero to do stuff, and so the writers are forced to come up with some flimsy explanation for why this particular guy has chosen to do things that are bad ... even when there are much better and easier ways to accomplish what he wants. Like ...
5Up -- Charles Muntz
During the first few minutes of Up, we are introduced to the bad guy, Charles Muntz. He's a famous explorer who falls into disrepute after allegations that his latest find -- a skeleton of a never-before-seen bird -- was a fake. Muntz vows to prove his detractors wrong by capturing said bird and disappears into the South American wilderness. Some 70 years later, an old guy named Carl ties a bunch of balloons to his house and floats away in it.
It's a complicated film.
Carl, his floating house and a Boy Scout named Russell somehow make it to South America and inadvertently befriend the same strange-looking bird Charles Muntz has been looking for all these years. As a result, Muntz sets Carl's house on fire, kidnaps Russell and then tries to kill them both by sending an army of talking dogs to shoot them in little planes.
There's a pun somewhere in here.
The Pointless Evil:
And don't say, "Because he was evil!" Even in terms of carrying out an evil plan to kidnap a rare bird, it doesn't make sense.
Things turn ugly at first because Muntz thinks Carl and Russell have come to steal the bird and rob him of his discovery, but Carl's story should have been verified once Muntz realized that he really did travel to South America in a floating house (if he was telling the truth about that, we'd believe literally anything else he said).
It's because he's old. Once you lose the ability to poop, everyone must pay.
But even if Carl and Russell turned out to be black market bird dealers or something, Muntz could have easily negotiated an arrangement with them, since he only needed the bird temporarily (that is, long enough to show it to the public). He was only making things harder for himself by antagonizing the only people who actually knew where the bird was.
And they brought along their entire kitchen as a warning.
Even if he was afraid Carl would take the bird to civilization and claim credit, he didn't need to worry -- all that would do is vindicate Muntz's discovery from 70 years earlier (remember, he had a skeleton of one -- he was unquestionably the bird's discoverer). Also, at one point Russell casually tells Muntz about the bird's weakness for chocolate bars (literally the only reason it befriended them) and he does exactly nothing with this information. It would have made a lot more sense to rig a few chocolate-laden traps, sit back and wait for the bird to wander into one.
"I don't understand. When do the flying dogs come in, exactly?"
It doesn't make sense from any angle for him to chase and terrorize the bird's friends for half the movie. If he had bothered thinking this through instead of instantly jumping to canine homicide schemes, he could have saved himself a lot of trouble and a lot of money in ammo and doggy parachutes.
Also, teaching dogs to pilot aircraft is arguably a more significant achievement than finding any bird.