If there's anything Igor has achieved so far in a month, been away from Vladmir's clutches. It has to be the fact that he's one step closer to taking everything that belonged to Vladmir. His intel from Vladmir's place has given him a very good piece of information that with just a strike, was enough to bring Vladmir down.
If anyone were to ask him the reason why he's doing this, trying to kill his brother. There's not an actual answer he has for what he's doing, all he knows is that Vladmir wasn't the right person to lead the Black shields, regardless of being family.
Using the disguise of being dead, he has gained the loyalty of the other few mafias and with the annual Mafia gathering that would be held in three days, he's going to disgrace Vladmir because right there, he will be publicly humiliated of the crimes he had committed against other gangs, which he's the one pulling the strings from the back.