Charles Austin Miller

Answer: Marcus may have had a change of heart after the fact; but it's more likely that, knowing John Wick's fearsome capabilities, Marcus believed John Wick was going to slaughter every one of the Russians (including Viggo), thereby erasing every reason for the contract. Marcus didn't want to be on the wrong side of John Wick's wrath if John actually did annihilate everyone. So, Marcus may have accepted the contract to lull Viggo into a false sense of security, thereby giving John Wick a further surprise advantage.

Charles Austin Miller

Answer: I feel it's because Marcus honestly believed he could double-cross Viggo with no consequences. And he almost did if not for Perkins. Basically he took the contract to subtlety help John, while also attempting to stack some money on the side.

Answer: Viggo put out a contract on John Wick (meaning Viggo offered a large bounty for somebody to kill John Wick). Marcus accepted the contract. The "Cardinal Rule" was that, if you accept the contract, you must kill your target. No ifs, ands, or buts. Marcus broke that Cardinal Rule because he did not kill John Wick when he had the chance.

Charles Austin Miller