Every monster movie needs a scientist character, and Dr. Serizawa fits the bill. Like many movie scientists, Serizawa's specialization isn't made clear. Is he a chemist? Physicist? Did he get his doctorate in fine arts? We aren't told. He just sciences his days away studying oxygen.
Movie scientists from the 1950s tended to come in two flavors: the mad scientist and the science hero. The mad scientist is typically detached from everyday life thanks to an obsession with their work. They don't see the inherent dangers of their experiments and, as a consequence, they often give birth to the movie's monster. Think Dr. Frankenstein from Frankenstein.
Serizawa does have some mad scientist qualities. He's obsessive about his work, and he does have a scary basement lab. However, he doesn't create Godzilla, and he's painfully aware of the dangers in his experiments. So, he can't be a card-carrying member of the mad scientist guild.
Does that make him a hero scientist? Again, not exactly. A typical hero scientist has an adventurous spirt. He'll study the danger head on and devise some genius solution to the problem. He believes science will save the world and so sciences his science for the good of humanity.
Again, this only partially describes Serizawa. He definitely comes to science with an eye for benefiting humanity. As he tells Emiko,
SERIZAWA: But I'm determined to find a use for the Oxygen Destroyer that will benefit society. Until then, I won't reveal its existence. That's why I told the reporter nothing.
But at the same time, he's hardly adventurous and he himself calls the Oxygen Destroyer "nothing but a weapon of mass destruction." That hardly seems in keeping with the world-saving hero scientist who trusts the scientific process to exclusively benefit humanity.
Then what are we to make of Serizawa, who mixes the tropes of the typical 1950s scientist character, thus making our job here more difficult? Let's dig deeper.
After fighting in World War II, Serizawa returned to Japan and began studying oxygen. In his studies, he discovered a new form of energy that removes oxygen from an aquatic environment, effectively killing all the living organisms and, for some reason, turning them into skeletons. (Best not think too hard about that last part; just let the movie movie.)
Rather than publish his research, Serizawa hides everything away in his basement. There, he continues to search for a way to use the energy to benefit humanity. He tells Ogata:
SERIZAWA: Ogata…if the Oxygen Destroyer is used even once, the politicians of the world won't stand idly by. They'll inevitably turn it into a weapon. A-bombs against A-bombs, H-bombs against H-bombs. As a scientist—no, as a human being—adding another terrifying weapon to humanity's arsenal is something I can't allow.
In this statement, his Oxygen Destroyer is directly related to nuclear weapons, so Serizawa's character is, in a sense, a critique of scientist and scientific endeavors that led to the development of nuclear weapons (and, by proxy, human suffering).
To be fair, the scientist who first made discoveries that would eventually led to nuclear arms probably couldn't see that far. When Martin Klaprothdiscovered uranium in 1789, it's simply not possible he could have foretold its use in history's most devastating weapons.
However, the scientists of the Manhattan Project definitely knew what they were about, and Serizawa's response would be that such a project is immoral because it brings suffering to humanity. Moral scientific endeavors are those that unequivocally bring benefits to the world. (Although how to foretell that research will only lead to good outcomes is a sticky issue without the use of a crystal ball.)
Put another way: when Robert Oppenheimer, director of research for the Manhattan Project, first saw a nuclear weapon detonated, he famously uttered words from the Bhagavad Gita, "Now I am become Death, the destroyer of worlds" (source). Serizawa's response to such a claim would be something along the lines of, "Yeah, but can you not?"
Serizawa's not only a critique of scientists. In a sense, he's also a type of moral one-upmanship. American scientists created the nuclear bomb and let it loose for the world to deal with, but Serizawa takes responsibility for his scientific discovery.
He agrees to use the Oxygen Destroyer to defeat Godzilla, but before he does, he burns all of his notes as "the only way to make sure it doesn't fall into the wrong hands." But Serizawa takes it a step further. He brings the Oxygen Destroyer into the ocean and detonates it while staying behind. The way he sees it, only by destroying the knowledge in his head can he truly prevent the weapon from being used against humanity.
So in the end, Serizawa is his own kind of scientist.
Like the mad scientist, he lost sight of his research and accidently created a devastating weapon. Like the hero scientist, he used his discovery to save humanity and defeat an unstoppable force. And the two kind of cancel each other out within Serizawa, resulting in a tragic end for this character.