Praise be to Allah.
Allah, may He be exalted, says (interpretation of the meaning):
“Because of that We ordained for the Children of Israel that if anyone killed a person not in retaliation of murder, or (and) to spread mischief in the land - it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind”
What is meant is killing a soul that Allah has made it forbidden to kill. Sa‘eed ibn Jubayr said concerning the words “it would be as if he killed all mankind”: Whoever regards it is permissible to shed the blood of a Muslim, it is as if he regards it is permissible to shed the blood of all people, and whoever regards it as forbidden to shed the blood of a Muslim, it is as if he regarded it as forbidden to shed the blood of all people.
Mujaahid said: Whoever kills a believing soul deliberately, Allah will make his recompense Hell, and Allah will be angry with him and will curse him, and will prepare for him a severe punishment. He is saying that if he killed all the people, that would not increase such a punishment.
Tafseer Ibn Katheer (3/93)
As-Sa‘di (may Allah have mercy on him) said: That is because he had no legitimate reason to do so, and no one should kill another person unless he has a legitimate reason to do so. If a person has the audacity to kill someone who did not deserve to be killed, it means that he does not differentiate between the one whom he killed and anyone else; rather he would do that every time his soul that is inclined towards evil prompts him to do so. Therefore his audacity in killing him is as if he killed all of mankind.
End quote from Tafseer as-Sa‘di (p. 229). See also: al-Jawaab al-Kaafi by Ibn al-Qayyim (p. 103)
With regard to the verse in which Allah, may He be exalted, says (interpretation of the meaning: “The recompense of those who wage war against Allah and His Messenger and do mischief in the land is only that they shall be killed or crucified or their hands and their feet be cut off on the opposite sides, or be exiled from the land” [al-Maa’idah 5:33],
Even though what these ignorant people and others like them do of spreading mischief on earth – and in fact all acts of sin and disobedience towards Allah – comes under the heading of spreading mischief on earth and corrupting it, this verse has a different meaning and refers to a different ruling that does not have to do with the topic of this question, because those referred to in this verse are those who want to spread mischief deliberately and intentionally, and they strive to do that on earth so as to spoil it, ruin it, destroy crops and livestock, and terrorise people who feel secure.
Moreover, this verse is speaking of a specific group of evildoers and mischief-makers on earth. Not everyone who spreads mischief on earth deliberately or transgresses against others comes under the ruling referred to in this verse.
Shaykh as-Sa‘di (may Allah have mercy on him) said:
Those who wage war against Allah and His Messenger are those who show enmity to them openly and spread mischief on earth by promoting disbelief, killing, seizing wealth and engaging in banditry.
The well-known view is that this verse has to do with rulings on bandits who attack people in the cities and in the wilderness or countryside, taking their wealth by force, killing them and terrorising them, so that people refuse to travel by the roads where they are, thus cutting off those routes.
Allah tells us that their recompense and punishment, when the hadd punishment is carried out on them, is that one of the things mentioned should be done to them.
End quote from Tafseer as-Sa‘di (p. 229)
With regard to what you mentioned about this person taking it upon himself to prescribe herbal medicines and remedies for diseases, if he does not have experience in what he is doing, and is not known to have knowledge in that field – if, as a result of his ignorance, he causes the death of an individual, then he must pay the diyah (blood money) because of the offence that he committed against another person as a result of his ignorance and transgression, but despite that he is not regarded as one who killed deliberately, as the verse mentions, and qisaas (legal retribution) is not stipulated in that case.
Al-Khattaabi (may Allah have mercy on him) said:
I do not know of any difference of opinion concerning the fact that if a healer oversteps the mark and the patient dies, he is liable, and the one who engages in a field of which he has no knowledge is regarded as having overstepped the mark. If his deed results in death of the patient, then he is liable for the diyah (blood money), but the qisaas (legal retribution) is waived in his case, because he did not do that without the permission of the patient.
End quote from Ma‘aalim as-Sunan (4/39)
And Allah knows best.