There is no fundamental difference between "man-made" drugs and "natural" drugs. Yes, weed is a plant and heroin is synthetic. But morphine, for example, comes from the opium poppy and LSD is synthetic. There is a huge difference in terms of how addictive and dangerous those substances are. Scopolamine also comes from a plant and is an intense deliriant that is highly toxic and is used to drug people in order to commit crimes in parts of South America.
Just like with alcohol, people sometimes commit terrible acts when on meth. That said, it does not cause that in every case, and legalizing it does not mean that those actions are excused. People will be able to get their hands on any drugs that they want, but legalizing them gives people the ability to treat addiction. While that may not solve those specific instances of crimes committed under the influence of methamphetamine, getting more people off these drugs through addiction therapy (instead of just arresting them and waiting for them to relapse) will be able to decrease these cases.
Jail the pusher and cure the user.
Legalizing will just loop and never ending addiction battle.
One cool idea is having it "legalized" but you can only get the drug with the doctor's approval. This could mean that existing addicts get prescriptions to the drug, and possibly someone can try a safe, small dose if they wish. However, this would be supervised by doctors who would be able to say whether the patient is in a mental health condition where it is safe to use the drug, overdose will be prevented, and they will be able to monitor the user. I do believe that there are problems with this system (a would-be-user could just save the drugs for later and sell them, not sure exactly how to prevent that), but having exact knowledge of who is buying/selling the drug will make it much safer than if more and more dealers continued to pop up. Ideally, selling black market drugs would be unprofitable so dealers would disappear altogether.