which of these 2 sentences are the meaning of that phrase?
1)if you want to live happily, you must not mind killing others
2)live your life and be ignorant about other people’s death.
I haven`t heard about this idiom before but there is one similar to yours
Live and let live and it is used to say that you should accept other people’s behaviour, even if it seems strange
“Live and let live,” is an idiom as Sandy rightly states.
“Live and let die,” is a film title that appears to sound like an idiom, but in reality is a spoof.
“Live and let die” is also a song by Guns’N’Roses. You know, one of those hair metal bands of the 1980s who thought it was cool to be self-centered. 
I can’t remember the song’s lyrics, but knowing the band, I’d interpret “Live and let die” like this: Live your life to the fullest (or live the way you want to), and let the others die (don’t care about others, especially if they don’t agree with your life style). Or is it about some murderous maniac? I think the answer lies within the lyrics.
Claudia
Good morning Claudia. If I remember correctly it was also the title song of the film,“Live and let die”.
Hello Kitosdad. Yup, by none other than Paul McCartney. 
Maybe Guns’N’Roses liked the film and theme song so much, they composed their song as a hommage?
Claudia
I searched the Internet and found out, it was Paul who composed the song, and Guns’N’Roses only made their own version of it. The lyrics seem to mean: When you were young, you were more tolerant (= live and let live), but as life goes on, you should become more jaded because of the competition at your job and the hardships of life, so you should get all grouchy and mean and ruthless (= live and let die). Nah, just kidding. The singer, either Paul or the Roses guy, wants to advice you not to let yourself get down. Yeah, that’s it!
Thank you all for your answers. I actually have heard it in a Song by Paul McCartney, and as Kitosdad correctly mentioned it was sang for a film by the same name; But I didn’t know Guns’N’Roses have sang it too!