The First Precept, “Refrain from taking the lives of sentient beings”, is the gateway to the Dharma. Once we’ve killed a sentient being (human or non-human) —directly or indirectly— it matters little if we violate the next four Precepts, as the dead cannot be harmed further by our lies, theft, sexual misconduct, or intoxication.
In fact, those who do not observe the First Precept are not yet on the Noble Path. This is not a personal opinion, or an exclusively Mahayana view. Asked to summarize His teaching, the Buddha said: “Refrain from harm. Do good. Purify the mind.” —Dharmapada XIV.5
One cannot ignore or skip over the very first step on the Path. Meditation and other Dharma practices are just vain self-indulgence for those who do not refrain from harming sentient beings.