Massive Lung Damage

Other answers have pointed out how mercury is dense. If you fell into a pool of mercury you would go splat rather than splash. That means it is difficult for your golem to envelop the victim without them simply floating to the top.

Instead I suggest your golem invade their body by perhaps only enveloping the head and then pumping mercury into the lungs until they stop working.

Of course two lungfuls of mercury weighs about the same as grown man. So the lungs will burst long before they fill up, and the victim will die in a much slower and more gruesome manner than you describe.

Collapsed Lungs

Other answers have pointed out how mercury is dense. If you fell into a pool of mercury you would go splat rather than splash. That means it is difficult for your golem to envelop the victim without them simply floating to the top.

Instead I suggest your golem invade their body by perhaps only enveloping the head and then pumping mercury into the lungs until they stop working.

Of course two lungfuls of mercury weighs about the same as grown man. So the lungs will burst long before they fill up, and the victim will die in a much slower and more gruesome manner than you describe.