Because before the Lord will deliver the Israelites from their enemies he requires them to forsake the false gods they have turned to instead of the true God. God asks Gideon to build a proper altar to the Lord and using the wood of the Asheral pole that he cut down, offer the second bull as a burnt offering.The risks involved were twofold. He risked offending his father and family and secondly the community at large. The punishment for his action according to community lawis death.
He did this at night as the chances of being seen are less.The night mission was on the face of it a sign of weakness because it showed that he was afraid of the community as well as his father but it was also a sign of faith because although he was afraid, he nevertheless carried out the mission as he trusted God and obeyed his command