First, brutal surge is an option, it doesn't replace your action surge. However, why do you think it is a terrible trade. It is the same damage and basically the same to hit probability.
It is bad, because you now only have a single chance to hit or miss. It is an all or nothing option, that might be ok, if you somehow get advantage on a single attack only.
The problem is however, that you give up up to 4 attacks to double damage on a single strike.
Maybe if you word it:
- you can use your action surge to double all damage from every attack you make during your current turn.
That at least helps you keep the same average damage. It is also a real advantage if you have access to a bonus action attack. Overall that should be a good option, especially if the campaign uses the optional rule to cleave, if you instantly bring an enemy from full to zero.
The autokill on a crit below 50hp is also not really a useful ability.
A crit at that level, especially if combined with the action surge should do about 50 damage anyway.... So the treshold is a bit low. It is also disappointing, if you roll high for damage and then notice that you had killed the monster automatically.
In 4e the executioner had a neat ability to kill the enemy if it was reduced to 20 hp or less instead of 0.
So I propose to change it to: if a critical hit from a weapon reduces the enemy's hp to 25 hp or less, the attack deals 50 extra damage of a type the weapon can deal.
(50 damage, because it should go through resistance, also just damage, because it should no invalidate abilities that trigger at 0 hp).