When landlords "invest" in the housing market, they are not making the system of providing housing for people better or more efficient. They are buying up a limited supply of properties that exist in desirable areas and then charging people for the right to use them plus a nice profit for themselves. This reduces the supply for a necessary good and drives up prices, making it profitable for landlords, and a massive, efficiency draining, example of rent-seeking for the system as a whole since the landlord's basically don't work and instead take a cut of what everyone else makes doing useful work.

If you invest in some predatory companies you might be investing in companies that do that, or might do some other predatory practice, but you can also just be putting money into a business so that it has more money to grow its operations, or invest in some new efficiency that makes them run better, and that then both returns a profit back to both of you and helps improve the system as a whole.

Think about it this way, when you retire, you are going to need money to sustain you for a long time after you stop being able to work, so while you're working, you need to save that money up. That money can just sit in your bank account doing nothing for anyone, or you can invest it in a business that lets them use those resources now and lets you get your retirement money back 30 years from now when you need it (though in reality that's spread across hundreds of companies to reduce risk). That's how investment can be a net benefit to society and make for a better use of resources, characteristics not present with landlords and housing investments.

No, not all investment is the same. look up the difference between rent and profit.

I don't think a bunch of people pitching in to fund a company is in and of itself a bad thing, but there are several considerations that are extensions of that that stray into unethical territory. foremost is the matter of fiduciary responsibility. When a company is publicly traded they have a legal responsibility to put money in the investor's pockets, and that shapes the behavior of that company in ways that can be very harmful. Intent shapes action, and the imperative to provide profits to investors changes whatever intention that company may have had when it was founded. It means if the company has a choice between fulfilling that imperative or doing something to reduce harm to the world around them they will always make the decision that fulfills the intention. Where if your intention is just to be the best at something, or to provide a service, you would make a different set of decisions. The biggest example that's particularly central in public consciousness right now is the health industry. Health insurance companies have the ability to ensure that their customers are well taken care of and that healthcare is accessible, but providing healthcare isn't the point. Providing profit to their shareholders is the point, so in every situation where the profit, and the doing the right thing, conflict they will always choose the former because that's the whole reason they're doing it in the first place. Even if the CEO wanted to lead the company in a more ethical direction they couldn't do so without courting legal action, if the investors believe their decisions aren't maximizing profits. Multiply this by time and companies gradually become worse, even if they started out great. Enshitification isn't just for the internet. Often this leads to unethical ends, as in the health insurance example where it causes thousands of deaths each year. A lot of it depends on whether the demand for something is fixed or elastic. Say you wanted to purchase a lot of something as an investment, if that thing is FunCo Pop figurines and you're hoarding them banking that they'll increase in value due to scarcity could be sold later at a markup. People can take or leave FunCo Pops, They can choose not to spend their money on your marked up collectibles. Hoarding them would be a dick move, but not necessarily unethical. If the thing you're buying up is water the landscape changes. People need water, every single person needs water. That demand is not elastic, people have to have it or they literally die. If you hoarded that resource so that you could sell it at a higher price, and that prices some people out of being able to access water, it's more than unethical. It's straight up wicked. Your intention isn't to provide water. It's to maximise your profits, and thus your decisions will always be guided by those priorities. It's nuanced. But not very difficult to understand. The world could change for the better, but the profit margins are too slim to make it a worthwhile goal for a savvy capitalist.