You're right to be scared of course - a software bug in a safety-critical system is a bug in a safety-critical system, no matter if it happens in a plane or in some piece of critical infrastructure. And yes, they have now patched this flaw, but most likely it's still a pile of old F0RTRAN code or something like that, that they're now stuck dealing with in some way. The notion that these sorts of systems are somehow less prone to having serious bugs in them is being revealed as a dangerous delusion.
No software is perfect, but the aerospace industry has standards as far as software validation/verification that mean airplane software is in general less bug-ridden than comparably complex low-level software written for less risk-averse industries. Hard to say if that applies to any particular chemical or energy plant as well.
I don't agree with this comment neither finds this speculation particularly interesting, but it's polite and on topic, and shouldn't be downvoted.
Yes but there's been no suggestion of malware introduced into the 737-MAX control software, as far as I've heard.