Why would you have to do that? I’m saying that characters have their own perspectives and they’re not always correct. Not only should that not be a controversial statement, but there are a ton of books, movies and TV that wouldn’t work if we could always trust that a character’s perceptions and beliefs were correct.
In the case of Thanos, if his methods were making paradises, surely the movie would have shown us some. Thanos is exclusively shown surrounded by corpses and destruction, and anywhere we see that he’s already left is a ruin. And in particular, if Gamora’s planet were actually a paradise (one wonders how Thanos would even know - he’s not exactly portrayed as someone who runs administrations or goes on tours of previous slaughters), surely he could have shown her that - at least in video. It would be far more convincing than him simply asserting it.
None of which is to say that Thanos’ solution isn’t effective, for some value of effective. If the problem is a shortage of nonrenewable resources, killing half of all life will absolutely help with that because there will be less demand. What I’m suggesting is that Thanos has either not bothered to consider (or written off) the knock-on consequences of that solution, the problems with a completely arbitrary halving, and so on and that a whole lot more people would naturally die as a result, in some places probably the entire civilization. That’s not the problem he’s set out to solve.