Firstly let me say I am no expert on the subject, just going on what I have read really.
So what is Satanism then? Apart from worshipping Satan....Satanism isn't always about worshipping Satan as a literal figure, In the case of LaVeyan satanism (The church of Satan Founded by Anton Szandor LavVey in San Fransisco in April 1966 (Walpugisnacht t be precise)) it is about worshipping the self (at least that's the public side of the CoS). it is about self indulgence and I guess total acceptance of every aspect of the psyche, The most important festival to a LaVeyan Satanist is his/her own birthday.
Then there are groups who do believe and worship a literal Satan, I believe the Temple of Set falls into this category,(I may be wrong because I do not know much about the ToS)The ToS was formed in 1975 at a time when the Church of Satan was having problems with internal allegations of corruption etc.
In the case of the ONA well they do believe in a literal Satan and advocate some very dodgy practices. Articles apparently penned by them openly advocate burglary, deep infiltration of society, Nazism and murder. Candidates for sacrifice are tested by their actions to see if they are "worthy" which acording to the group they would be if they have a flawed character for example cowards, traitors and those who prey on the elderly are cited as examples. In some cases the ONA extends this to evangelist Christians. The group has published quite a large number of articles on this and other subjects which you can easily find on the net. I'm not sure if the group still exists, my only experience of them is through their writings which I first read in the mid 90's
I always thought of Satanism as a rebellious, low key thing that people did just to be "cool". A way to get back at the church and its doctrine. For a lot of the followers of the Church Of Satan I think that is exactly right, they were drawn to it I guess as a clear way of rejecting Christian or other religious doctrine that made them feel guilt for what is natural human response or action.
I know when I was reading up on this kind of thing I was going through a rebellious phase and I gained a certain amount of strength from what I was reading. Though it didn't take me long to realise that it wasn't for me at all. There have always been to an extent a more sinister underside, a less public movement that seems to have a connection with Arianism (though not always) and other extremist groups.
Is it a serious religion?Yes it is, many of the followers are very serious in their beliefs.
Why worship something that is ultimately just after chaos and death?It's mainly about self empowerment, creating yourself as a god in a way. Giving the practitioner to do anything they want and being totally free of guilt, Thats not to say that All Satanists are engaged in criminal activity, Far from it, Some of the people I know who are Satanists are the most Moral, Law abiding people I know.
And why do all the satanic websites use pentacles and pagan symbolism for their own symbols?Same reason we use them, because they fit a purpose, they hold some form of meaning to the group, ok it may be a twisted meaning from what was originally meant etc. like the Nazi version of the swastika. the use of the inverse pentagram by the church of Satan fits well with the goat head of Baphomet that is used in their sigil, the horns being drawn in the two points, then the ears in the next two and the snout in the lowest point. maybe that's over simplifying the answer but it's all I can think really, the symbols used have meaning for them.
A few weblinks to read through, BE Warned some articles are not for the feint hearted.
The Official Site of the Church Of SatanThe Temple Of Set Official Site A Mirror of the ONA Official Site An Expose article about the ONASociety of The Dark Lilly a site with lots of articles on Left Hand Path groups.