Joseph Smith: does his reliance on the Bible excuse his immoral behavior?
One defense of Joseph's immoral behavior is that he used the Bible as his guide and moral compass.
Joseph knew the Bible was imperfect
Joseph Smith and the Book of Mormon taught that the Bible was corrupted.
Joseph Smith created an inspired translation of the Bible, correcting passages through 'revelation' from God (see Joseph Smith Translation or JST). This was an opportunity for God/Joseph to correct wrong and immoral teachings.
Joseph had a direct revelatory channel to God
Joseph allegedly received many revelations from God during his time as prophet, including admonitions and corrections. Joseph had a direct channel to God, so there was no need to rely on the Bible as a moral compass.
God gave Joseph Smith revelations about trivial matters. For example, God gave incredibly detailed instructions on building and financing the Nauvoo mansion (D&C 124). God could have easily given more important revelations. God evidently cares more about baptism at age 8 than polygamous marriage at 14.
Joseph received specific instruction regarding polygamy
The Book of Mormon taught against polygamy with the exception to raise up seed at God's command. The BOM specifically denounced David and Solomon's practice of polygamy.
Also regarding polygamy, Joseph reportedly prayed to God and received the problematic revelation that is now D&C 132. Joseph did not need to rely on the Bible because God himself could answer Joseph's question. This was another opportunity for God/Joseph to correct wrong and immoral teachings in the Bible. Why didn't they?
Tldr: Joseph did not need to rely on the corrupted Bible because he had direct access to God. Joseph's "revelation" (not the Bible) justified some of his immoral behavior including polygamy. The Bible should not be used as an excuse for Joseph's immoral behavior.