do you think that itachi didnt really fully obtain the mangekyou sharingan? maybe he wants to wait for sasuki to grow up and be stronger so he can face his beloved brother and complete the mangekyou sharingan tech. because sasuki is his brother therefor they might have the same abilities so maybe killing a stronger person with the sharingan can make the mangekyou sharingan tech go higher. because both are brothers they might aquire thru bloodline that they are both the best with the sharingan because sasuki is not strong enough itachi wouldnt be fully good with mangekyou sharingan so killing a stronger sharingan user can make the tech more powerful. remember on the chapt it said only few sharigan users can have master the ability. and technically itachi didnt really kill the whole clan, sasuki is still alive.