There was this manga that I forgot the title of, with a main character who was a writer, I think. He longed to die and his blood was literally poison. I believe that there was also a coffin that he liked laying inside, either pretending and wishing that he was dead, or he just liked laying in it; I cant remember.
An adventure in another world with cute girls by your side and video game-like powers–sounds like an anime fan’s dream, right? Not so for melancholic author Osamu Dazai, who would quite literally prefer to drop dead. Video games haven’t even been invented yet when he gets yanked into another world in 1948. Really, all the fantastical adventure he keeps running into is just getting in the way of his poetic dream of finding the perfect place to die. But no matter how much he risks his hide, everything seems to keep turning out okay.
The story opens with the main character attempting a double suicide with a woman, but is hit by a truck before he can "properly" kill himself. He wakes up in another world and is proclaimed a hero by a priestess, except his level is still 1. He has no interest in such hero business and goes out to try to die.
He encounters a treant that tries to kill him, but it doesn't work because his blood is poisonous. Possibly due to him continually munching on sleeping pills.
And yes, he tries to sleep in a coffin. I haven't read past this but I suspect it's a common thing.
Found by searching mangaupdates for the tags