From a game on OGS. In the end black played A and those was heavly killed

Click Here To Show Diagram Code

[go]$$Bc
$$ X X O . . |
$$ O X X . . |
$$ X X X . . |
$$ X X O O O |
$$ O O X X X |
$$ . O X . O |
$$ . O X . a |
$$ ----------+[/go]

Black should play s5?

Yes, I saw that now, why he did not do that and I not play better against it is probably because we both are beginners I guess.
(But I really need to stop playing like that)

Org game is here

http://online-go.com/game/1193124

Actually, this position is not at all like seki. If a position is seki, neither player can put their opponent in atari without putting himself in atari. In the position you show, (if we ignore the fact that b can capture white as Abyssinica pointed out) white can put black in atari at S1 or S2, so if not for S5, black would simply be dead. You can read an explanation about seki here: http://senseis.xmp.net/?Seki

_________________
Patience, grasshopper.

Leaving aside the specifics of this case, learning about seki you need to learn the difference between temporary seki and real seki.

It is not seki just because the inside situation is seki. Let's suppose for just a moment that the situation in the corner was locally a seki (that would be a different arrangement of stones, but let that pass for a moment. Would this be a seki? No, because the white stones just above could be captured and after that, with outside liberties, black could capture the white stones inside.

Why am I saying this? Precisely because it is one of the things beginners have to learn to look for (when learning about seki). In other words, asking not just whether internally there were seki possibilities but also considering whether the outside groups were in fact all safely alive would have led to seeing the move S5.

I love when I open up someone else's game, think something could become seki, and when I ask them about it they point out the fact that some stones on the outside were dead that I completely did not notice.