Jesus often referred to the religious crowd as "serpents". He also called himself Living Water and invited us to drink deeply from Him. If he is Water that gives life, is there not water (poison) that brings death? For me, I take from Mark that we will handle the religious crowd (serpents) and not be hurt in our souls, and that even though they teach poison, it will not get into us and damage us so long as we walk with Jesus and drink from Him (as his disciples whom he spoke the command to did).
13 "But go and learn what this means: ' I DESIRE COMPASSION,AND NOT SACRIFICE,' for I did not come to call the righteous, but sinners."