The trouble with 'realistic' combat systems is that the players die too easily. The attraction of D&D and (games like it) is that PC are hard to take out. The problem is that the one shot kills on sentries (by sneaking up) and or sniper fire is also impossible as per the rules.
In the op's presented scenario I would allow an instant kill, simply because it makes for a good story line but it would be pure DM fiat.
Thing is with the minion rules, one shot kills on sentries now becomes a possiblility, of course not all sentries will be minions. The way I look at it, given the move to more cinematic storytelling in 4e, many more 'realistic' things will be possible, though they may require the expenditure of additional resources.
Again in the op scenario, spending an action point to get a second surprise action and sneak attack could have taken care of the guard and as another pointed out, being between the guard and the bell could have given the rogue another action.
In the op's presented scenario I would allow an instant kill, simply because it makes for a good story line but it would be pure DM fiat.
Thing is with the minion rules, one shot kills on sentries now becomes a possiblility, of course not all sentries will be minions. The way I look at it, given the move to more cinematic storytelling in 4e, many more 'realistic' things will be possible, though they may require the expenditure of additional resources.
Again in the op scenario, spending an action point to get a second surprise action and sneak attack could have taken care of the guard and as another pointed out, being between the guard and the bell could have given the rogue another action.