JOEL - Here are the relevant scriptures:

"Again, let my servant John C. Bennett help you in your labor in sending my word to the kings and people of the earth, and stand by you, even you my servant Joseph Smith, in the hour of affliction; and his reward shall not fail if he receive counsel.
And for his love he shall be great, for he shall be mine if he do this, saith the Lord. I have seen the work which he hath done, which I accept if he continue, and will crown him with blessings and great glory."(D&C 124:16-17)

I am sure you noticed all the "if" phrases in this instruction that God gave. "if he receive counsel"; "if he do this"; "if he continue". There are very few people on this earth who are completely evil, but it sounds to me like God knew about his questionable character, but was still willing to give him a chance to serve and be blessed....."if".

God can call someone to a position or to perform some task, but the fulfilment of that assignment is always subject to man's own agency. And God's reward of blessings for that person is conditional upon their performance and faithfulness.
No one on this earth is perfect, and God knows that, but we are all He has to work with, so He gives us assignments and commandments and we have our agency to choose to do or not do them. And whether good or bad we all learn from the experience.
Similar things happened in the Bible. Why did God give to Moses the ten commandments for His chosen people to follow, only to find out later that they turned to worshipping idols, and He had to take back the commandments and have them live a lesser law? Didn't God know they would fall into sinful ways? He could have saved Moses a trip up that mountain for the tablets.
It's hard to understand how God can be all-knowing and yet still allow imperfect men to carry out His will. If He knows for certain that they are going to fail then why command them to do it in the first place?
How does free agency come into play if God already knows the outcome? This kind of question is hard for us to understand. It's almost the same as asking "If God is so all-powerful and creator of all things, can He create a rock too big for him to lift?"
"For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD" (Isa 55:8)