Takshaka cannot be called evil, just going by 2 incidents. Brahma modified the curse of Kadru, that only evil serpants will die, but not virtuous.
Thus, those who survived the snake-sacrifice must be virtuous.
Let us revisit the 2 acts of Takshaka that were mentioned by the OP.
Takshaka killed Parikshit
Takshaka had stolen ear rings from Utanka.
1. Takshaka killed Parikshit
a) It was Sringin, who cursed 'That sinful wretch of a monarch who hath placed a dead snake on the shoulders of my lean and old parent, that insulter of Brahmanas and tarnisher of the fame of the Kurus, shall be taken within seven nights hence to the regions of Yama (Death) by the snake Takshaka, the powerful king of serpents, stimulated thereto by the strength of my words!'
Thus, it is clear Takshaka was propelled by the power of curse uttered by Sringin. How can we find fault with Takshaka?
b) After hearing the commencement of snake-sacrifice, Takshaka took refuge at Indra, who assured him safety by saying:
'O prince of snakes, O Takshaka, here thou hast no fear from that
snake-sacrifice. The Grandsire (Brahma) was pacified by me for thy sake.
Therefore, thou hast no fear. Let this fear of thy heart be allayed.'
c) Vasuki was 2nd incharge of snakes after Adiseha. He gets his sister Jaratkaru married to a sage with the same name Jaratkaru, to beget Astika, as per the direction of Brahma.
d) It was Vasuki, who through his sister implores Astika, nephew of Vasuki, to stop the snake sacrifice. And, Astika stops it.
2. Takshaka had stolen ear rings from Utanka.
Yes, he had stolen ear rings from Utanka. And, upon the fire emanated in Naga loka, he returned those ear rings. So the matter should have ended there.
However, why should Utanka, a learned brAhmin, should allow anger/revenge take over him? Being propelled by the curse of Kadru, he had become instrumental in instigation of King Janamejaya to take up the snake sacrifice.
So we cannot blame Takshaka to be evil, in my view.