Did Sha'ul himself offer the Korban? He was not a Kohen!
Rashi: A Zar may offer on a Bamah (Zevachim 120b). Radak - Bamos were permitted then (Zevachim 112b).
Why did Sha'ul not wait for Shmuel?
Alshich: Sha'ul was unsure if Shmuel meant seven days me'Es la'Es (until the eighth day, at the same time of day as now), or the end of the seventh day. Normally he would have been stringent, but Pelishtim had gathered? so he offered b'Erev (towards the end of) day seven. 1
Malbim (8, 9): (a) There was great danger, like the previous verses said. (b) Shmuel told him to seven days. Sha'ul thought that part of [the last] day is like the entire day. (c) People were leaving. Sha'ul feared to be left alone. (d) He thought that Shmuel meant only not to start the war until he comes, but he did not forbid to offer Korbanos.
Alshich explains that Sha'ul offered at the end of day seven, and Shmuel came on day eight. "K'Chaloso" implies right after he finished! Will he say that Sha'ul finished offering if at the very end of the night, and Shmuel came at dawn? Perhaps the opinion that one may offer at night on a Bamah (Zevachim 120a), permits to finish offering the next day, and Shmuel came right after. However, 'b'Erev Yom Shevi'i ' does not imply at night. (PF)
