Greed
In all honesty, Tristan knew that the best way to get more Blood Essence was still through battle. Not only was this exploit faster, it would also yield him much more Blood Essence than absorbing from these creatures.
This attempt of his trying to get rare creatures' blood was only thought to be an extra Blood Essence 'revenue' on the sideline. After all, with more Blood Essence he gained, he could quickly get more upgrades to further ensure his survival.
[Upgrade Blood Synthesis to stage 3 requires 50,000 Blood Essence]
[Upgrade Blood Seal to stage 4 requires 50,000 Blood Essence]
[Upgrade Blood Monarch Body to stage 6 requires 100,000 Blood Essence]
These three upgrades which each had its own outrageous cost would still go a long way into making him the strongest person on this planet. Therefore, it was only natural that for Tristan, Blood Essence was the highest and most prioritized commodity.