[RPG] Can a cleric choose to channel damage or healing in a case by case basis

Necromancy is not inherently evil

Necromancy spells are only characterized by their use of negative energy; negative energy is entropic in nature but is not a moral force. The inflict line of spells, for example, aren't evil.

But...

Necromancy does have some evil spells. Thankfully, they'll helpfully be labeled [Evil] in the spell descriptions!

Spells associated with particular alignments are indicated by the chaotic, evil, good, and lawful descriptors in their spell descriptions.

Per the Cleric class description, the Cleric cannot use spells with an alignment component opposed to their own; Good Clerics can't use [Evil] spells, Evil Clerics can't use [Good] ones. When in doubt, just check the spell's description!

Further, Channel Energy does not affect non-spontaneous spellcasting. While Channel Energy determines what spells a Cleric may cast without preparing, it doesn't affect spells that the Cleric memorizes ahead of time and does not otherwise restrict their casting list, though their alignment may do so (see above).

By RAW, the spells Stabilize and Bleed don't specify that they channel energy, so they don't. It can be thought of as magically stitching closed, or ripping open, a wound; something to stop the excessive bleeding. Also consider that Stabilize is only on the Cleric/Druid spell list, while Bleed is on both the Cleric/Druid and Wizard/Sorcerer spell lists. If you flipped the effects, Wizards and Sorcerers could stabilize your Dhampir as well as Clerics, and only Clerics or Druids could magically bleed him. You could house-rule this easily if you prefer this flavour.

In terms of creatures other than your Dhampir, creatures with the Undead subtype don't have a Constitution score, so die immediately at 0hp. Bleed and Stabilize, therefore, would never normally be a consideration for actual Undead.