Many people will take their UBI and immediately spend it on drugs, alcohol, gambling, or bling, while ignoring the monthly rent, the electric bill, buying groceries for the children, etc.
Do you have evidence this is true for welfare and other checks, or is it just how you feel? I suspect you've never been in the heartbreaking situation (which I'm glad you haven't experienced it!) of having so little income that you have to decide between food and the electric bill. I'm sure there are some outliers that can't be helped and will spend on drugs but you need to understand this is a small minority compared to all poor people.
So the various government agencies will continue to expand and spend even more money on housing, food, medical care, etc. The UBI won't even make a dent in entitlement budgets. Instead, it will become "free money" to be squandered on a thousand other things besides basic human needs.
Again, citation? Has anyone's plan specifically said "We will grow government larger and larger"? Most of the proposals I've seen have been the opposite; if you make a fair tax system (stop giving tax handouts to the rich) and implement UBI instead of the hodgepodge of programs we have no (SS, medicare, medicaid, etc.), we'd save billions by eliminating duplicate administrative costs.
Now my concern is that many people are employed by the federal government, so the real cost will be all the people worried about losing their jobs and becoming poor. But if there's UBI, they won't lose their home if laid off. And, its possible we could pivot many of these jobs to other agencies -- for example, more workers in the justice dept to reduce the time we wait for hearings/court cases, or to the VA to get caught up on paperwork and get veterans help, or even dept of the interior and let them clean up state and national parks or become EPA inspectors to actually enforce our laws. Random ideas here, but the point is that government will likely be reduced, and worst case, be about the save size but massive amounts of people repurposed to things that need to get done but aren't under the current bureaucracy.
Anyone who doesn't think it won't happen need only look at inner city schools in the U.S. In theory, every child should be getting meals at home thanks to government SNAP benefits to their parents or guardians. In practice, schools give many kids a free breakfast and lunch every school day, and even give them food bags to take home for the weekend, because Mom or Dad can't be bothered to buy food for the kids with the SNAP money. Where does the money go? No one knows or even attempts to find out. They just give the kids free food and cross their fingers.
What do you mean "Where does that money go?". I don't even know where you got this from.
As someone that was personally on SNAP in the past (long story, but basically as a new college instructor, you actually make so little money that I qualified for SNAP for a while. True story.), I can tell you that it is not a check in the mail of free money. You get a debit card that is pre-loaded with a small amount of money (a maximum of $200 per month for an individual; I challenge you to keep your food budget under $200 per month = about $7 a day. You do get more money for each dependent you have, but it's a small increase.). This card can ONLY be used by stores that accept SNAP, and it is restricted to ONLY purchase food items. For example, you cannot swipe your SNAP card to purchase lottery tickets or alcohol. You're not even allowed to buy "prepared food" (meaning like food you'd get from a restaurant; so you have to buy frozen foods or canned foods only, and cook at home).
Anyone on SNAP that can't feed their kids is probably running up against that roughly $7 per day limit. Even if you double it to $15/day for a family, can you spend $15 per day consistently? A pound of chicken is pushing $10. Milk is a few dollars. A box of cereal is pushing $5. You easily go over $15 with a single meal, let alone 3 meals a day. You have to be creative and buy the cheapest (and therefore crappiest and unhealthiest) food available to make it stretch. And even then it probably doesn't go far enough. Even if you do make it work, you're eating terrible quality foods, not enough vegetables, lots of things with corn syrup and sugar -- over time, it contributes to health problems which cause even more side effects since the person can't afford their healthcare. Since many Republican governors refused to expand Medicaid, there's a large amount of people that don't get Medicaid but private insurance won't cover them because they're assumed in the healthcare law to have Medicaid. They're trapped in a "donut hole" so to speak. It's not just old people's prescriptions.
School lunches are not evidence of people abusing SNAP benefits; rather, they are evidence that SNAP doesn't go far enough to actually help people. It's hard to say if a program is successful or not when many people (I'm mostly thinking of you, Congressional Republicans) don't even allow the program a chance to work as intended. I think a UBI rolling up SNAP, welfare, and other benefits, that is sufficiently increase to be a true living wage (or at least close to it) is absolutely what we should shoot for, and seriously try.
The UBI will not change human nature. It will instead become one of the biggest entitlement boondoggles in the history of civilization.
In my experience, human nature often means people and strangers helping each other out. We're social creatures and like to live in "tribes" where we can help each other. I feel bad for anyone that is so convinced that humans are nothing but laziness and evil when it clearly isn't true. Not that there aren't problems, and some people are bad apples, but on the whole, its not near as dark and gloomy as you suggest. We'll be fine as a civilization as long as we actually start doing the right thing and helping each other, instead of rationalizing why its ok to let people suffer and die.