boy cut lines, the same or not?

Does this mean to cut the veins in your wrists and bleed to death?

Basically, yes. I think wrists are the most common place for this, but not the only one.

lines mean veins?

Because we have two ways of saying this in different contexts: (1) [suicide] slit one’s wrist; (2) [boredom] (be about to/want to) open a vein.

No, Masha, veins are usually not called lines.

https://books.google.com/ngrams/graph?content=cut+her+veins%2C+opened+her+veins&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Ccut%20her%20veins%3B%2Cc0%3B.t1%3B%2Copened%20her%20veins%3B%2Cc0#t1%3B%2Ccut%20her%20veins%3B%2Cc0%3B.t1%3B%2Copened%20her%20veins%3B%2Cc0

https://books.google.com/ngrams/graph?content=cut+her+veins%2Copened+her+veins%2C+open+*+vein%2C+slit+*+wrist&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Ccut%20her%20veins%3B%2Cc0%3B.t1%3B%2Copened%20her%20veins%3B%2Cc0%3B.t2%3B%2Copen%20%2A%20vein%3B%2Cc0%3B%2Cs0%3B%3Bopen%20a%20vein%3B%2Cc0%3B%3Bopen%20the%20vein%3B%2Cc0%3B%3Bopen%20another%20vein%3B%2Cc0%3B%3Bopen%20this%20vein%3B%2Cc0%3B%3Bopen%20his%20vein%3B%2Cc0%3B%3Bopen%20any%20vein%3B%2Cc0%3B%3Bopen%20every%20vein%3B%2Cc0%3B.t2%3B%2Cslit%20%2A%20wrist%3B%2Cc0%3B%2Cs0%3B%3Bslit%20his%20wrist%3B%2Cc0%3B%3Bslit%20her%20wrist%3B%2Cc0%3B%3Bslit%20my%20wrist%3B%2Cc0%3B%3Bslit%20your%20wrist%3B%2Cc0

so what this?
a boy cut lines on both of his arms cause he wants to die.

Yes, for example.

hurrah

A boy SLITS BOTH WRISTS because he wants to die.

grumbler, гений. Просто и скромно - гений!