Question: Why did Walt save Jesse, when he wanted him dead?
Answer: He originally wanted him dead but over the year or so he was on the run he lost his family and his money and realised the horrible person he had become. He thought Jesse was already dead but when Pete and Badger told hi the blue meth was still in circulation he realised Jesse was being forced to cook and decided to save him because he was no longer the ruthless Heisenberg but was now back to being Walter Whiter.
Say My Name - S5-E7
Question: Did Walt plan on killing Mike? If so, why? It seems like he just did it in a fit of rage, but then why did he take his gun?
Answer: As you stated, it was in a fit of rage, he regretted it right afterwards.
Answer: For one thing the engine running the pump isn't that loud. The characters have conversations at a normal volume without having to scream at each other. Even if the conductors could hear it, there is no reason they should care or be suspicious. It just sounds like an engine running in the distance and they don't care enough to look for what is making the noise. At that very moment there is also a kid riding a dirt bike in the area. Engine noises are a very common thing.
BaconIsMyBFF