I won't comment on Ukraine specifically, however, the military industrial complexes of Western countries aggressively lobby for forever wars so that they can profit endlessly.
At this point nothing is clean in the current economy in the world. The current world is build by warlords (British Empire), robber baron (Rocketfeller, Ford), drug dealers (HSBC), or descendant of them. But some of us may have fiduciary duties to our family or dependants, or student loans, this is only reason that big corp can get people working for them.
If you don't have any of these responsibilities, then do something else that is meaningful for your life.
In this context "defence" likely means "defence industry" and not "defending your country" per say.
Lots of people regard working on weapons used to kill other people as unethical. For example, you could be a pacifist [0]. Or from the military standpoint you be concerned about Just War [1] and whether the weapons you build will be used for something you see as Just or not (say in your example selling arms to the wrong side). Theres a bunch of others like this.
Because you are building stuff to kill people. Even the doublespeak of naming it defence is atrocious. It might be justifiable in some cases like the current weapon shipments to Ukraine, but not if they are used in something like the Iraqi war. And you definitely wont be the one who decides where the weapon you helped to make is used.
Yes, you are building stuff to kill people who are killing you (and democratic Western world in general). Quite ethical, if you ask me. And most weapons in the world are not used to kill people — they are used to deter people from killing people.
And, if you choose to not work for autocratic countries, you have a choice in saying who will use the weapons.
It's funny how democratic Western world gives themselves the ethical right to kill others while fighting for “truth” and turning a blind eye to their own war crimes, which is ultimately a reflection of the rhetoric of their opponents.
This impeccable confidence in the rightness is simply amazing.
You're effectively a part of western world, and it doesn't sound like you're turning a blind eye to western war crimes. Moreover, they are covered in big newspapers, remembered and regretted for decades, and often lead to criminal cases and/or resignations.
I can 100% assure you that it's not like that in Russia, and that's the difference between modern west and other superpowers, past and present.
Even with the Iraqi war, we're talking about a regime that killed hundreds of thousands of its political opponents [1], gassed civilians [2], and engaged in genocide [3]. It is really not clear to me that stern talking and maybe sanctions while essentially abandoning victims to be slaughtered by the regime is the moral choice here. See also: Syria.
Plus, when it gets to the actual intervention, tech minimises casualties, making the war shorter and enabling more precise strikes. The US did not need to indiscriminately shell cities like Russia does in its current war.
Worldwide militarism is a problem, it's a shame that governments still see force as a legitimate means of policy, and investment in force capabilities rather than mutually beneficial peaceful ones. Such waste, such idiocy.