The first Precept in the book name Buddhist morality by Prof. Phra Thepsophon p.96 Said about the first precepts that In observing the first precept, the Buddhist undertakes to abstain from killing, causing to kill, or sanctioning the destruction of a living being. ’Living being (pana)’ implies anything that has life, from a tiny insect up to man .This precept applies to all creatures. It forbids the killing of man as well as animal.
To be successful in the observance of the first precept, he has to purify his intention by developing and cultivating his in the observance of first precept, he has to purify the his intention by developing and cultivating loving-kindness (metta) and compassion (karuna) in his mind. Once the intention is motivated by loving-kindness and compassion it naturally manifests itself in right action and right speech which are in conformity with the precept. That is why morality or precept is described in the Visuddhimagga as follows, "What is precept? Such states as the intention of one who abstains from life-taking and forth, or of one who fulfils his set so duties
Suffice it to say that observance of the first precept, abstaining from killing, is facilitated by an intention full of loving-kindness and compassion. without these two virtues in the mind, one would find it difficult to observe the precept. Thus the practice of loving-kindness and compassion should accompany the observance of the precept.
What follows from the foregoing discussion is that loving-kindness(metta) and compassion(karuna) are two virtues necessary and essential for observing the first precept. The positive side loving-kindness of non-killing is the development of and compassion, which are the psychological bases of non-violence. Motivated by these two mental is purified of hatred and, qualities, man's intention as a consequence, his desire for violence subsides and he automatically observes the first precept of abstaining from killing living beings. As it is said by the buddha
There, someone avoids the killing of living beings, and abstains from it. Without stick and sword, he lives scrupulous, full of loving-kindness and anxious for the welfare of all living beings