Erik Selvig, a doctor of literature or meteorology—Luo Xia couldn't remember which—was the scientist who had helped Loki use the Tesseract to open a portal for the Chitauri.
Although he had been under hypnosis at the time, it was clear that Dr. Selvig was extremely talented. You couldn't just grab any random person and expect them to open a portal big enough for an entire army.
Luo Xia had never considered killing him in advance to stop Loki from opening the portal. There might not be as many scientists on Earth capable of doing it as there were stray dogs, but there were probably more than ten. He couldn't exactly slaughter them all.
Besides, the Chitauri were Thanos's lackeys. As long as Thanos had no intention of giving up the Infinity Stones on Earth, the Chitauri would find some other way to cause trouble there, even without a portal.
So this was a battle that had to be fought.
The only real question was how large it would be and when it would happen. If it was merely a surprise attack through a single portal, Earth's current strength might just barely be enough to hold out. But if the Chitauri invaded on a massive scale and unleashed war in every corner of the planet, humanity would have almost no chance of fighting back.
War was inevitable. All they could do was prepare for it. They had to make the most of every advantage—time, terrain, and manpower—and prepare as thoroughly as possible, doing everything they could to close the gap in equipment and troop strength.
Luo Xia still remembered watching Guardians of the Galaxy before he crossed over. When the Kree's enormous warship descended from the sky, the Nova Corps' starfighters could only form an energy net and try to stop