As per the story of Kartikeya, the god destroyed the demon Tarakasura.Some stories claim that he even killed the demon's brothers,Surapadman and another demon,whose name I have forgotten.However,other than this story,I have not come across any other tales of Kartikeya vanquishing demons.Has Kartikeya kill any other demons too?
गांगेयस्ताम्रचूडश्च ब्रह्मचारी शिखिध्वजः । तारकारिरुमापुत्रः क्रौंचारिश्च षडाननः ॥२॥
Gaangeyas-Taamra-Cuuddashca Brahmacaarii Shikhi-Dhvajah | Taaraka-Arir-Umaa-Putrah Krauncaa-Rishca Ssaddaananah ||2||
Meaning: 2.1: (Salutations to Sri Kartikeya) Who is Loved by Mother Ganga and His FollowerTamrachuda, Who is Celebate and has Peacock as His Emblem, 2.2: Who is the Enemy of Tarakasura and Krauncasura, Who is the Son of Devi Uma and has Six Faces.
Kartikeya is a Hindu deity also known as Skanda and Murugan and Subhramanya.(कार्तिकेय , स्कन्द , मुरुगन , सुब्रह्मण्य)
We find mentioning of Skanda Or Kartikeya in Salya Parva Of Great Epic Mahabharata.(शल्य पर्व. महाभारत) book 9 Chapter 46 , and his stories about fighting and killing of many daityas.
These are excerpts -:
Thousands of daityas, O king, were burnt with the flames that issued from Skanda's dart, while others breathed their last, terrified by the roars of Skanda.
The adorable Skanda, inflamed with rage, quickly slew the son of daitya chief (Bali) along with his younger brother.
He then, in that battle, slew Mahisha who was surrounded by eight padmas of daityas. He next slew Tripada who was surrounded by a 1,000 ajutas of daityas. The puissant Skanda then slew Hradodara, who was surrounded by tennikharvas of daityas, Then Bali's son Vana of great might, getting upon the Kraunca mountain, battled with the celestial host. Possessed of great intelligence, the great generalissimo Skanda rushed against that foe of the gods. From fear of Kartikeya, he took shelter within the Kraunca mountain. Inflamed with rage, the adorable Kartikeya then pierced that mountain with that dart given him by Agni. The mountain was called Kraunca (crane) because of the sound it always produced resembled the cry of a crane. That mountain was variegated with shala trees. The apes and elephants on it were affrighted. The birds that had their abode on it rose up and wheeled around in the welkin. The snakes began to dart down its sides. It resounded also with the cries of leopards and bears in large numbers that ran hither and thither in fear. Other forests on it rang with the cries of hundreds upon hundreds of animals.Sharabhas and lions suddenly ran out. In consequence of all this that mountain, though it was reduced to a very pitiable plight, still assumed a very beautiful aspect. The vidyadharas dwelling on its summits soared into the air. The kinnaras also became very anxious, distracted by the fear caused by the fall of Skanda's dart. The daityas then, by hundreds and thousands, came out of that blazing mountain, all clad in beautiful ornaments and garlands.
It seems like there is an evidence/story in Skanda Purana about Kartikeya killing many demons along with a prominent demon called Pralamb.
Below is how Skanda purana describe the incidence:
A demon named Pralamb had escaped unharmed in the battle fought between the deities and demons. As the deities were busy installing Shivalingas at different places, Pralamb started to torment their families in their absence. The deities got this information from Kumud who appeared at the time when the deities were digging up a reservoir near 'Siddheshwar' temple.All the deities requested Kartikeya to do something. Kartikeya released his fiery weapon, Shakti in the direction of Pralamb. Shakti pierced the heart of earth and went right up to Patal loka where Pralamb lived with the surviving demons. All the demons were killed within few moments.
So, yes, Kartikeya may be more known for killing Tarakasur, he also killed several other demons too, as per Skanda Purana.