this is how i will kill someone

All at once too, or the body would absorb it. Hell, it might absorb it anyways

Best to use an air compressor with a needle rather than a syringe. It’s a little harder to get into the vein and a bit louder but 15-20min of pumping and results are guaranteed.

just fake a gas leak. No one will ask any questions

They likely will. Modern safety and code+the fact that when things explode people almost always ask questions. Fire teams trace the flash point and they’ll find the line. Signs of tampering are obvious. Natural gas is mixed with a chemical so it has a distinct smell. Questions abound. Now- every victim has a “perfect kill.” There is no one method. An elderly person who is known poor quality DIY repair person or in an older ran down home perhaps has works. With the syringe- if they do have an embolism and were young and healthy- an autopsy will likely reveal an injection site and a lack of toxins or drugs in the system raises questions as well as- “where is the needle if they od’d?” You need to know your targets habits and rep, their general medical state can help too. A junky od-ing? Not a lot of questions there if there’s no sign of struggle. Befriending then can help but the more there is linking you to them the more likely you are to be investigated. Motive is your #1 place to get..

... caught. Part of why serial killers can be so hard to catch. Without knowing theor patern there is no obvious motive. Once you know their MO- that still leaves a lot of random people who could fit depending on what it is. Random targets in random places are hard to track if you don’t leave records of your travel and traveling/being off the grid are normal for you. Bragging and calling cards are the next big catch followed by complacency and careless mistakes. Your best bet in general though is a tailored approach that relies on things about the target so that the cause and circumstances of death fit their life and don’t seem out of the blue.

When I said a gas leak I did not mean to explode them. I meant to deprive them of oxygen in their sleep. Also signs of tampering are only obvious when you don't know what you are doing. Making it look like the line end attached to their stove got damaged by the stove being moved at the wrong angle would not draw many questions especially if they were found dead in their sleep.

Dubious. As I said- I’m not saying it couldn’t work. You’d need very specific circumstances for it to work. Modern gas lines are designed to be relatively robust. Otherwise we would see a lot more gas leaks if a little bump was all it took to damage one. It has to be a substantial whack. So the person in question must both have been able to physical move a stove with such force to do so- and have had some reason to do so- like perhaps if they just remodeled or moved. Then one wonders how they would have not noticed the smell at concentrations that high, and if they have close neighbors of some type no one else could notice. Then there’s issues of how one would access said line without it being suspicious or leaving some evidence, and the question of if their home is air right enough to allow the gas to displace enough air to suffocate them. There are a lot of variables is what I was saying- so in the right cases it could work- but it’s a hard sell IMHO.