Hello guys. i need advice to improve character's quality.

I take the assasin role on the build, but I frequently die over the maps. Things getting

horrible in later games. I need your advice on the gem skill, equipment and passive tree

etc. Reply appreciated.

Well, first thing, your build lacks consistency.
What would be your main skill ? Herald of Agony, Bladefall, Static Strike, Blight ?...
Choose one and build around. Get a 4-link and put good supports for it.
With your current stuff, you have bonuses for attacks, so Bladefall and Blight are out (they are spells).

Herald of Agony builds are currently very powerful and unexpensive. Search a build that offers a budget/beginner version and duplicate it.
For instance, you'd need a skill to poison so as to summon the Crawler, so Cobra Lash or Venom Gyre come to mind. So you need two claws or two daggers with poison stats, like Wasp's Nest or Bino's Kitchen.
Then, in a 4 link, put : Herald of Agony + Minion Damage + Damage on Full Life + Lesser Poison, for instance.

If you prefer Static Strike, that's another build completely. You can mix both but it's more complicated.