Why didn't Vishwamitra kill those mighty demons himself?
The simple answer is: He's now a brāhmaṇa, no longer a kṣatriya, so cannot wield weapons.
Swami Chandrasekarendra Saraswati in the book Hindu Dharma: The Universal Way of Life § explains as below.
The Brahmin must be conversant with the fourteen branches of the Vedic lore. He must be proficient even in Gandharva-veda or music and must be acquainted with agricultural science, construction of houses, etc. At the same time he must give instructions in these subjects to pupils from the appropriate castes. His own vocation is the study of the Vedas and he must have no other source of income.
Visvamitra was the master of Dhanurveda (military science). When he performed sacrifices, the demons Subahu and Marica tried to play havoc with them. Though a great warrior himself, he did not try to drive away the demons himself. Instead, he brought Rama and Laksmana for the purpose. Visvamitra thereafter gave the instruction to the two in the use of astras and sastras.
If the Brahmin is asked, "Do you know to wield a knife? " he must be able to answer, "Yes, I know". If he is asked, "Do you know to draw and paint" again he must say, "Yes". But he cannot wield a knife or become an artist to earn his livelihood. All he can do is to learn these arts and teach others the same according to their caste. He is permitted to receive a daksina to maintain himself and he must be contented with it however small the sum may be. The Brahmin's speciality, his true vocation, is Vedic learning.
[Hindu Dharma » Varna Dharma For Universal Well-Being » Brahmins are not a Privileged Caste]
§ An older and copyright-free version of this book is available for free here.