Could God be honored by a sinful act (human sacrifice), if it was to fulfill a vow?
No. Fulfilling a vow was a high priority in ancient times (Lev.5:4; Deut. 23:21–23; Joshua 9:19; Eccl. 5:1–5) and Jephthah apparently didn’t want to be humiliated by reversing his promise. But God had outlawed human sacrifice (Lev. 18:21; 20:1–5). God disapproves of sin, even one committed to fulfill a vow.