Why Mike throw his handgun after dumping the Turk at the restaurant? He can throw it somewhere such as river, lake or physically dismantle.

He was instructed by Clemenza to drop the gun before leaving the restaurant. He had already wrapped in in special tape that would resist fingerprints.

There was no time to 'physically dismantle' or drive to a lake to throw it into. Michael had to be immediately brought to wherever he would embark on the trip to Italy.

Leaving it at the scene was the best thing to do with the gun.

Apple

Paratroopers, when a shooting occurs in the US, the murder weapon is the strongest evidence that can be used against the accused. If the police find the gun on the accused, and ballistic tests prove that it was the murder weapon, he will be convicted. That's why Michael had to drop the gun immediately. That way, if a policeman entered the restaurant almost immediately and arrested him, he wouldn't have the gun in his possession. And fewer restaurant patrons would have seen him with the gun.

Oh, I see. It seemed very different with our country policemen detective methods. Though our police need evidences for accuring suspect, 'imaginary' evidences is enough. For this case, the cop and the Turk are dead, only Mike is still alive at the murder scene. And only these 3 guys had a dinner together. No more trajectory test, No more fingerprint analysis for our country policemen, there can be no doubt that Mike is murder. If he didnt admit it, OK, our policemen have "methods" to let he talk.

Yes - it eliminates the possibility of him being caught with the firearm in his possession, and the remote possibility he might be drawn into a second shooting - or providing an excuse for policeman to shoot him in the head, because he is armed.

It also means the murder weapon will be found at the scene, so a detective can't "invent" a weapon and place it on anybody

It's obvius that is better be caught without the weapon in your hand, even if it's just in front of your feet

"Imaginary evidence" is sometimes enough in this country, too, if the authorities want to get you badly enough.

...i too was very uneasy reading the section of the novel where michael is being prepped for the assassination of sollozzo and mccluskey and being told he must drop the gun at the scene. even with the special tape preventing his fingerprints from being left on the weapon, could he not have accidently put his prints on the gun when he fumbled for it in the restaurant washroom? way too risky. and all those witnesses. i kept thinking, no take that gun with you michael corleone and toss it into the river. but to leave it right there on the floor with two dead bodies - one of whom is the chief of police - felt like a mistake. get rid of the gun. if they trust michael to go all the way through with this massively important killing, can he not be trusted to dispose of a pistol?

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In the novel he's told that they can square things with witnesses, etc., but they can't do anything if the cops find him in possession of the gun.

...michael's brother sonny used him ruthlessly to assassinate the corleone enemies. i was wondering why michael was so willing to risk his life and freedom that way. my guess is he was still in soldier-mode, having recently arrived back in america from active combat duty. in his mind going to kill mccluskey and sollozzo was no different from killing enemies in the battlefield and he saw it as a military operation. plus he was still young, the bravado of youth...

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What makes a person willing to kill to exact revenge or to protect something? Michael had such a nature. To that extent he was just like Vito when Vito was younger. Sonny was like that only emotionally. He didn't kill as a strategy. Michael and Vito had substantive reasons to kill. Remember, in the novel, Michael tells Vito that it's not just out of revenge for Appolonia and Sonny that he is doing this. He told him that it's also the right thing to do. So, his nature is to view murder as right.

Don't fool yourself into thinking that. Mike, just like Vito, took EVERYTHING personally and revenge was done for personal reasons, not because it was "right".

And when they killed Parlie the gun was also left in the car. I remember the novel said it was to terrify the potential betrayer in the family.

The gun was left in the car because the car was in a remote place, and there were no witnesses to the shooting. So, there was no reason to take the gun away. Leaving it there was the safest thing to do.

It probably had tape on it just like the one Clemenza gave Mike. But, of course, the novel says that Lampone threw the gun into a nearby swamp. That whole canoli thing in the film is interesting.

Did Connie eat antidote beforehand, just wondering?

That's in GFIII. She ate just barely a bit.