why didnt she kill him after sex?
i never understood why he agreed to go there in the first place
and why she didnt just kill him? someone please explain
The same reason Leonidas did not try to kill Xerxes in the first film. Under a banner of truce each side temporarily agrees to cease hostilities to talk. During that time period promises, trust and honor was much more important. Since there was no phone system and communicating by messengers is time consuming two opposing parties had to to be able to trust each other enough to meet and talk knowing the other would not try to kill them.
If Themistocles had killed Artemisia he would have dishonored Greece, Athens, and himself to such a degree Athens would have likely had him executed or at least exiled (which back then was virtually the same thing.) It would have destroyed any trust and further attempts to negotiate.
Same with Artemisia. She would have broken not only her word, but the word of Xerxes whom she represented. No one would trust him and he would have had her executed.