An opinion can be valid or invalid based on how the opinion in question can be used as a tool by a sentient being to carve out more knowledge that sentient others can use to form their own opinions.
Hypothetical person A has the opinion that one object grouped with an identical yet separate other of its kind results in a net total of three of the aforementioned objects, where one and three respectively correspond to representations of amounts of x and xxx. When this hypothetical person tries to extrapolate more knowledge from this opinion, the person is unable to create any infallable logical structures as their base opinion used to create further knowledge is factually incorrect and therefore is only valid as an example of an opinion that is invalid.
To say that the worth of a human life is entirely subjective is to make a lot of assumptions of the viewed utility of a human. It all depends on the epistemological foundation one is working from; a human life can range from having incalculable value of net utility to having a negative net utility.
It all depends on the foundation one is working from and convincing other sentient beings that the foundations and specific chain of logical conclusions from those foundations that lend to the opinions one most ascribes to are the ones that others should ascribe to as well. If one is unable to convince others to hold one's opinions through the most rational of arguments from one's chosen foundations, then one's opinion is invalid for one's purpose. A lone opinion has no utility outside of its originator until it is adopted by another. It can be transferred to another through a device that can store information given functioning encoding and decoding operations, but without one or the other of those functions, an opinion's existence contributes only to an increase in entropy by virtue of being.
Wait. What? Aaaaa. Get out of my head you fucking clown. Damn the DMT, that shit is wild. Fucking 2d sillhouettes of jester beings made of light cast against and infinite darkness mocking me and letting me know I shall be forever alone if I don't stop thinking this way. Fuck the jesters. There does exist pure objectivity. There is maths behind this world. Beautiful and currently intractably complex maths binding all together. There is the fog of opinion that stands in stark contrast to pure truth, and we simple beings have never been able to see pure veritas as we are stuck. We will get there. We will know beauty. We need as many minds working on it as possible. We need it.