Why, exactly, did they have to destroy the island to get away from the "dinosaur." If the thing could've chased them across the lake, it would have. I suppose it made for an "exciting" finale, but there's no logical reason for it. Unless you consider "Lizard must die!" logical.
Hypothetically they are preparing this planet for colonization, and killing the dinosaurs rendered it safe for human life. Of course it's still incredibly stupid because A) they are assuming that island was the only place on an entire planet that held such giants, B) if they are correct then the dinosaurs seem confined to the island, and thus humans would only have to avoid that one area, and C) now humans definitely shouldn't go to that island, as it is now radioactive. So yeah, "Lizard must die!" might actually be the sane explanation.
Remember, this is back when the government didn't let people know about the radiation from bombs to try and keep them from getting too scared about the whole "Cold War" thing, back when a nuclear attack could happen at any moment. (Hey, everyone thought it was all heroic when they did it in The Dark Knight Rises)
The "we have to make camp" scene is odd. After the perilous (ha) snake attack, Ralph says they should stop until daybreak, I assume due to concerns about nocturnal predators. Fine. But why bed down in the area they must know is crawling with killer alien snakes? And finally, despite the original reason for them having stopped (i.e. Ralph's apparent concern about attacks), all four team members participate in the shelter-building with nobody posted on guard duty, meaning everyone could've been attacked and killed by something sneaking up on them at any time while they were focused on work. They do take turns watching, but only after the shelter is constructed.