what I'm trying to do: When someone passes trough the pressure plate it will lock both doors and open the floodgate. When they are finally dead i'll close the floodgate and open the doors.
The problem is that pressure plates don't work like that. When the pressure plate is stepped on it sends a signal to activate whatever it's connected to. This signal will cause both doors and floodgates to open. Doors open instantly, floodgates open with a 100 step delay. When the creature on the pressure plate steps off the pressure plate, the pressure plate will deactivate, sending signals to close the doors and floodgates.
What you're trying to do is have the pressure plate activate a latching system. To do this you need to use water logic. You need to build a side chamber connected by a door to a source of water. The pressure plate you have here will open said door to let water into the side chamber. The side chamber will contain two pressure plates, one set to trigger when the chamber is full of water, and one set to trigger when it isn't. The pressure plate set to trigger when water is present will be linked to the floodgate that lets water into your drowning chamber. The one set to trigger when water isn't present will be linked to the doors leading in and out of the drowning chamber.
Now when a creature steps on the pressure plate, it will cause the floodgate letting water into the side chamber to open, flooding the side chamber with water. This will un-trigger the pressure plate linked to the doors in and out of the drowning chamber, locking them, and trigger the pressure plate linked to the floodgate that opens to let water into your drowning chamber.
Finally, you can have another floodgate attached to the side chamber that drains water out of it. This you link to a manual lever in your fortress so you can drain water out of the latch after everything has drowned to reset the mechanism.