It also seems as if there is some confusion in what was sinful and what was substitutionary. Of course, the act of murder is not the ground of justification. The murder was not committed by the vicar. Christ lay down His life for His friends (John 15:13). It was not sinful for Him to lay down His life, so there is no contradiction to the law. Love fulfills the law (Gal. 5:14) which love He demonstrated in all His sufferings.

As far as the command and time to repent, I agree: a command does not translate to an ability. So why give time to repent when it is only God who grants repentance (2 Tim. 2:25)? This question, however, doesn't even work from the Arminian perspective, because if God is all knowing, He already knows that a person won't repent. Therefore, even if God is not sovereign, giving time could only be showing them temporal mercy because there won't be a changed outcome.

At the end of the day, we should simply live by faith and not try to wrap our minds around God-- that is a losing proposition. Let God be God-- sovereign, immutable, infinite... The question "why" is not ours to ask.