The Gemora in Shabbos (110a) cites the verse in Koheles [10:8]: One who breaks through a stone wall will be bitten by a snake. This is referring to someone who does not heed the words of the Sages. One is not permitted to scoff at the decrees of the Rabbis. The Gemora in Eruvin states that one who transgresses the words of the Chachamim is liable to death at the hand of Heaven.
Rashi in Avoda Zarah (27b) states that even if he will be given medicine for this snake bite and will be healed, other snakes will come and he will eventually die.
The Maharal explains: The Rabbis goal was to erect a fence to safeguard the commandments of the Torah. One who negates these decrees is causing a breakdown for the mitzvos of the Torah. This is why we deal with him so harshly.
Rabbeinu Yonah explains why one who violates a Rabbinical decree is dealt with in a stricter manner than one who transgressed a Torah commandment. One who violates a Biblical prohibition respects the law, but he is motivated by his physical desires to sin. He is not rebuffing his obligation, rather it can be regarded as a momentary slip in his observance. One who violates a Rabbinical enactment does so because of a lack of regard for their decrees. He belittles them on account that they were not written in the Torah and there is no real necessity to keep them. He is rejecting his obligation and therefore deserving of death.