Objectives & Reinforcements
Basic foundation for mission objectives in place; in this instance, Assassination. Kill the target, then the exit becomes available -- but if you were spotted or heard reinforcements are called and search the map for you.
Currently, reinforcements are endless. The idea behind this is you're not supposed to stand and fight, you're supposed to run. Assassination missions are heavily stacked against you by thematic design, so letting you blast the reinforcements and then be left with an queit map seems like the opposite of what is intended. If you stick around after murdering an important person, you're going to be overwhelmed by armed & armoured guads.
These reinforcements also have body armour, making them tougher to fight if you're not packing an armour-piercing weapon. So, if you're packing stealth weapons, you really CAN'T slug it out.
I'm hoping the contrast will be balanced, so being more covert lets you get further before an alarm but unable to deal with the consequences as well, but being more aggressive lets you deal with them better but triggering them sooner. If that makes sense.